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Risk Analysis, Vol. 22, No. 6, 2002

A New Approach to Risk Evaluation and Management:Risk-Based, Precaution-Based, and Discourse-BasedStrategies1

Andreas Klinke2 and Ortwin Renn3

1. INTRODUCTION

We define risks as the possibility that human ac-tions or events lead to consequences that harm as-pects of things that human beings value (Kates &Kasperson, 1983; Hohenemser, Kates, & Slovic,1983).4 This definition implies that the severity of ex-perienced harm depends on the causal relationshipbetween a stimulus (human activity or event) and theconsequences. If we take a nonfatalistic viewpoint,consequences can be altered either by modifying theinitiating activity or event or by mitigating the im-pacts. Therefore, risk is both an analytic and a nor-mative concept. If the vast majority of human beingsassess potential consequences as unwelcome or un-

1 The risk evaluation and classification concept was developed bythe German Advisory Council on Global Change (WBGU) in itsannual report in 1998 about global environmental risks. OrtwinRenn as a member and Andreas Klinke as an associate researcherhave been the main contributors to the new risk concept.

2 Project Manager and Coordinator Risk Research Projects De-partment, Technology, Society, Environmental Economics, Cen-ter of Technology Assessment in Baden-Wurttemberg, Indus-triestrasse 5, 70565 Stuttgart, Germany; tel.: +49 (0) 711 9063-288;fax: +49 (0)711 9063-175; [email protected].

3 Chair of the Board of Directors, Center of Technology Assess-ment in Baden-Wurttemberg, Industriestrasse 5, 70565 Stuttgart,Germany; tel.: +49 (0)711 9063-160; fax: +49 (0)711 9063-175;[email protected].

4 In economic theory, risk refers to both gains and losses. Sincewe are dealing here with risks to the environment and humanhealth, we believe that the confinement to negatively evaluatedconsequences is more in line with the average understanding ofrisk in this context. One should note, however, that the labelingof consequences as positive or negative refers to genuine socialjudgments and cannot be derived from the nature of the hazarditself.

desirable, society is coerced to avoid, to reduce, or, atleast, to control risks.

To reduce or control risks, social institutions areformed to evaluate and manage risks. In this context,we understand risk evaluation as the process by whichsocietal institutions such as agencies, social groupswithin society, or individuals determine the accept-ability of a given risk. If a risk is judged as unac-ceptable, adequate measures for risk reduction arerequired. The process of reducing the risks to a leveldeemed acceptable by society and to assure control,monitoring, and public communication is coveredunder the term “risk management” (Kolluru, 1995;Zimmerman, 1986:436). The debate on how to evalu-ate and manage risks focuses on three major strategies(Stirling, 1999).

1. Risk-based approaches, including numericalthresholds (quantitative safety goals, exposurelimits, standards, etc.).

2. Reduction activities derived from the appli-cation of the precautionary principle (exam-ples are ALARA, i.e., as low as reasonablyachievable, BACT, i.e., best available controltechnology, containment in time and space, orconstant monitoring of potential side effects).

3. Standards derived from discursive processessuch as roundtables, deliberative rule making,mediation, or citizen panels.

Past experiences demonstrate that there is nosimple recipe for evaluating and managing risks. Inview of worldwide divergent preferences, variationsin interests and values, and very few, if any, univer-sally applicable moral principles, risks must be con-sidered as heterogeneous phenomena that preclude

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1072 Klinke and Renn

standardized evaluation and handling. At the sametime, however, risk management and policy would beoverstrained if each risky activity required its ownstrategy of risk evaluation and management. Whatrisk managers need is a concept for evaluation andmanagement that on the one hand ensures integrationof social diversity and multidisciplinary approachesand, on the other hand, allows for institutional rou-tines and easy-to-implement protocols.

Our main thesis in this article will be to offer a newclassification of risk types and management strategiesthat promises scientific accuracy, a reflection of socialdiversity, and political feasibility. The proposal we de-scribe in this article certainly needs adjustment to thepolitical and regulatory culture in which it will be used,but it might offer some elementary building blocksfor a variety of culturally adapted pathways to riskevaluation and management. The article explains ourapproach to risk evaluation, classification, and man-agement. The authors developed this proposal duringtheir tenure at the German Government’s AdvisoryCouncil on Global Change (WBGU, 2000).5 The pro-posal includes criteria for evaluating risks, a classifi-cation of risk types and risk management strategies,and suggestions for institutional procedures aimedat implementation and testing. A crucial element ofthe proposal is the integration of analytic-deliberativeprocesses into the regulatory framework (Stern &Fineberg, 1996).6 The extent to which deliberationand analysis need to be mobilized is contingent onthe type and circumstances of the risk under consid-eration. To support this reasoning, the article startswith an overview of the current issues in risk anal-ysis and management (Section 2). We then use thefive major issues identified in Section 2 as the guid-ing elements for presenting our analytical approachtoward rational risk decision making, including riskevaluation and risk classification (Section 3). Section 4deals with corresponding risk management strategiesthat can be derived from the preceding risk classifica-tion. Section 5 provides further considerations aboutthe applicability of analytic-deliberative procedureswithin risk management. The last section summarizesthe results of this article and points to the main issuesthat need to be resolved.

5 The WBGU is an independent advisory board for the Germangovernment. It includes 12 members from different scientific dis-ciplines and an equal number of personal assistants.

6 The term “analytic-deliberative process” was introduced in therisk community by the National Research Council of the UnitedStates.

2. CONTROVERSIAL ISSUES IN RISKMANAGEMENT

Risk analysis has become a routine procedurein assessing, evaluating, and managing harm to hu-mans and the environment. However, there has beena fierce debate over the legitimate role of risk analy-sis for regulatory decision making. The debate centersaround five major themes.

1. Realism versus constructivism.2. The relevance of public concerns revealed

through perception studies as criteria for riskregulation.

3. The appropriate handling of uncertainty inrisk assessments.

4. The legitimate role of “science-based” versus“precaution-based” management approaches.

5. The optimal integration of analytic and delib-erative processes.

The following sections will first introduce each ofthese five themes in more detail and develop somemajor insights for risk evaluation and management.These insights will then serve as heuristic tools for thepresentation and explanation of our own approach torisk evaluation and management.

2.1. Realism Versus Constructivism

The first major debate in the risk managementcommunity touches on the philosophical questionof constructivism versus realism. For a philosoph-ical review of the two “risk camps,” see Shrader-Frechette (1991), Bradbury (1989), and Clarke andShort (1993:379–382). Many risk scholars have ques-tioned the possibility of conducting objective analysesof risk. The issue here is whether technical risk esti-mates represent “objective” probabilities of harm orreflect only conventions of an elite group of profes-sional risk assessors that may claim no more degreeof validity or universality than competing estimates ofstakeholder groups or the lay public. Reviews of theimplications of a constructivist versus a realist conceptof risk can be found in Bradbury (1989) and Renn(1992). A pronounced constructivist approach can befound in Hillgartner (1992), Luhmann (1993), Adams(1995), or in a recent German book by K. Japp, Sozi-ologische Risikotheorie (1996). Realist perspectivesin the social sciences on risk and environment canbe found in Catton (1980), Dunlap (1980), Dickens(1992), and Rosa (1998).

Risk-Based, Precaution-Based, and Discourse-Based Strategies 1073

The constructivist camp claims that risk assess-ments constitute mental constructions that can bechecked at best against standards of consistency, co-hesion, and internal conventions of logical deduction.However, these assessments cannot assume any va-lidity outside of the group’s logical framework. Incontrast, the realist camp is convinced that techni-cal estimates of risk constitute true representationsof observable hazards that can and will affect peo-ple as predicted by the calculated results regardlessof the beliefs or convictions of the analysts involved.For many technical experts, the philosophical posi-tion of constructivism seems absurd; for many socialscientists and philosophers, the realism of the scien-tists seem naive at best and imperialist at worst. Thedebate has produced severe repercussions on the is-sue of risk evaluation and management: if risk assess-ments are nothing but social constructions, they haveno more normative validity for guiding regulatory ac-tion than stakeholder estimates or public perceptions.If they do represent the objective reality, risk man-agers should use them as the guiding principles forhandling risks. Other competing estimates by stake-holders or individuals may serve as additional inputfor strategic purposes, for example, to honor publicconcerns, to enlighten one’s communication with thepublic, and to reconcile conflicts among stakeholders.Risk management agencies are well advised to reflecton this debate. To focus on the objectivist perspectiveonly ignores the social processing of risk information;to rely only on the constructivist perspective may leadto more fatalities and other adverse effects than nec-essary under the condition that there are only lim-ited societal resources available for risk reduction (cf.Shrader-Fredehtte, 1991).

2.2. Public Concerns as Criteria for Risk Regulation

The second major debate is closely linked to thefirst. It refers to the issue of inclusion. Many socialscientists, in particular those who claim that risk isa social construction rather than a representation ofreal hazards, have argued in favor of integrating pub-lic concerns into the regulatory decision process (e.g.,Freudenberg & Pastor, 1992). The key issue here ispublic involvement in defining tolerable risk levels(Lynn, 1990). Since it is the people, so goes the argu-ment, who are affected by the potential harm of tech-nologies or other risk-inducing activities, it should betheir prerogative to determine the level of risk thatthey judge tolerable for themselves and their commu-nity (Webler, 1999; Harrison & Hoberg, 1994). Many

technical experts have argued forcefully against thisproposition: they argue that sensational press cover-age and intuitive biases may misguide public percep-tions. Ignorance or misperceptions should not gov-ern the priorities of risk management. Spending largesums of money for reducing minor risks that fuel pub-lic concerns and ignoring risks that fail to attract pub-lic attention may lead to a larger number of fatali-ties than necessary (cf. Leonard & Zeckhauser, 1986;Cross, 1992; Okrent, 1996). If one spends a fixed bud-get in proportion to lives saved, the public at largewould benefit the most.

The debate on the legitimate role of risk percep-tion in evaluating and managing risks has been goingon for the last two or three decades.7 Defining risk as acombination of hazard and outrage, as Peter Sandmansuggested, has been the fragile but prevailing com-promise in this debate, at least in the United States(Sandmann, 1988). Although the formula of “riskequals to hazard and outrage” does not provide anyclue of how to combine scientific assessments withpublic perceptions, it provides a conceptual, thoughoften ritual, foundation for the general attitude ofrisk management agencies. Again, the debate has notcome to an end (and probably will never come to anend), but any reasonable risk management approachneeds to address the question of inclusion.

2.3. The Appropriate Handling of Uncertaintyin Risk Assessments

The third debate in the professional risk commu-nity centers around the handling of uncertainty (vanAsselt, 2000). This topic has been one of the most pop-ular themes in the professional community for manyyears, but is has reemerged in recent time for severalreasons.

� Philosophers of science and risk have pointedout that the term “uncertainty” implies a port-folio of different aspects that are often ne-glected or amalgamated in risk analysis (cf.Funtowicz & Ravetz, 1990).

� Advances in mathematics and modeling havemade it possible to be more precise in calcu-lating variability among humans or other risktargets. The general convention of using safetyfactors of 10 or 100 as a means to include in-terindividual variation can now be replaced

7 Cf. the special 1998 volume of Reliability Engineering and SystemSafety, Special Issue on Risk Perception versus Risk Analysis,59(1).


by more precise and adequate modeling tech-niques (Hattis & Minkowitz, 1997).

� The new global risks such as climate changeor sea-level rise have turned the attentionof many analysts to issues of indeterminacy,stochastic effects, and nonlinear relationships.Although these topics are not new to the riskcommunity, they have triggered a new debateover the umbrella term “uncertainty” and howit should be decomposed and handled (Wynne,1992; Lave & Dowlatabadi, 1993).

Several suggestions have been made in the pastyears to distinguish several components of uncer-tainty. It is obvious that probabilities themselves rep-resent only an approximation to predict uncertainevents. These predictions are characterized, however,by additional components of uncertainty. It seemsprudent to include these other uncertainty compo-nents in one’s risk management procedure. Whichother components should be included? There is noestablished classification of uncertainty in the litera-ture (see von Hasselt, 2000, for a review; cf. Stirling,1998:102). Authors use different terms and descrip-tions, such as incertitude, variability, indeterminacy,ignorance, lack of knowledge, and others. A new riskmanagement approach should look into these differ-ent types of uncertainty and find appropriate ways ofdealing with them.

2.4. “Risk-Based” Versus “Precaution-Based”Management Approaches

The fourth debate picks up the question of how toevaluate uncertainties and transfers this problem intothe domain of risk management. As stated in Section1, the assessment of risks implies a normative man-date. Most people feel a moral obligation to preventharm to human beings and the environment. Risk an-alysts are asked to provide the necessary scientific in-put to assist risk managers in this task. Since thereare more risks in the world than society could han-dle at the same time, risk management always impliesthe task of setting priorities. The conventional solu-tion to this problem has been to design risk reductionpolicies in proportion to the severity of the poten-tial effects (Crouch & Wilson, 1982; Mazur, 1985).Severity has been operationalized as a linear combi-nation of magnitude of harm and probability of oc-currence. Risk-risk comparisons constitute the mostappropriate instrument in this perspective for set-ting risk management priorities (cf. Merkhofer, 1987;Wilson & Crouch, 1987; Cohen, 1991).

The most significant argument against the pro-portional risk management approach comes from theanalysis of uncertainty (Cooke, 1991; Marcus, 1988).Most risk data constitute aggregate results over largesegments of the population and long-time duration(Funtowicz & Ravetz, 1987). In addition, there areproblems of extrapolation and dealing with randomevents and ignorance. The risk community has beentrying to respond to this challenge by sharpening itsanalytical tools, particularly with respect to character-izing and expressing uncertainties. Progress has beenmade, particularly in modeling variability, but someissues, such as the treatment of indeterminacies, re-main unresolved.

An alternative approach has been to change man-agement strategies and add new perspectives to theway of coping with uncertainties. Rather than in-vesting all efforts to gain more knowledge aboutthe different components of uncertainty, one can tryto develop better ways to live or co-exist with un-certainties and ignorance. The new key words hereare: resilience, vulnerability management, robust re-sponse strategies, and similar concepts (Collingridge,1996; WBGU, 2000). According to these concepts,risk management is driven by making the social sys-tem more adaptive to surprises and, at the sametime, allowing only those human activities or inter-ventions that can be managed even in extreme situa-tions (regardless of the probability of such extremes tooccur).

In the risk management literature these twoapproaches have been labeled science-based andprecaution-based strategies (cf. O’Riordan &Cameron, 1994; Stirling, 1999; Klinke & Renn,2001). This labeling is rather problematic since thesecond approach, which rests on precaution andresilience, needs at least as much scientific input asthe first approach (cf. Charnley & Elliott, 2000). Weprefer the term “risk-based strategy” for the firstapproach. With the denotation of “risk” it becomesclear that management relies on the numericalassessment of probabilities and potential damages,while the denotation of “precaution” implies prudenthandling of uncertain or highly vulnerable situations.Over the last few years, advocates of risk-basedand precaution-based approaches have launched afierce debate over the legitimacy of each of theirapproaches. Advocates of the risk-based approachargue that precautionary strategies ignore scientificresults and lead to arbitrary regulatory decisions(Cross, 1996). The advocates of the precautionaryapproach have argued that precaution does not


automatically mean banning substances or activitiesbut would imply a gradual, step-by-step diffusion ofrisky activities or technologies until more knowledgeand experience is accumulated (Bennett, 2000). Tonobody’s surprise, environmental groups have ralliedaround the precautionary approach, while mostindustrial and commercial groups have been fightingfor the risk-based approach.

Again, the issue is not resolved, and the debatehas become even more pronounced with the defeat ofthe European Community in the recent WTO settle-ment of hormones in beef. The European Communityfailed in providing sufficient evidence that the precau-tionary approach could justify the restriction of im-ported beef treated with hormones. In January 2000,the European Union issued a White Paper on the pre-cautionary principle in which it confirmed its posi-tion of applying this principle to hazard management(European Commission, 2000). Since the applicationof the precautionary approach may have lasting im-plications and repercussions on regulatory decisionsand international trade, the stakes in this debate arenot merely theoretical or academic. Depending on theapproach chosen, regulatory actions may vary consid-erably and shape economic competitiveness, publichealth levels, and environmental quality.

2.5. The Optimal Integration of Analyticand Deliberative Processes

The fifth and last debate in the risk communityfocuses on the legitimate role of deliberation in riskanalysis and management. A highly debated reportby the National Research Council emphasized therequirement for a combination of assessment anddialogue, which the authors framed the “analytic-deliberative” approach (Stern & Fineberg, 1996).Analytic-deliberative processes encompass proce-dures that are constructed to provide a synthesis ofscientific expertise and value orientations. Delibera-tion is the term that many authors adopted to high-light the style and nature of a discursive process thatthey believe is capable of dealing with the problemsof uncertainty and ambiguity in risk management(Webler & Tuler, 1999). The word “deliberation” im-plies equality among the participants, peer review asa means for verifying understandings (i.e., holdingknowledge claims up to public scrutiny), and an orien-tation toward resolving conflicts in consensual ratherthan adversarial ways (cf. Habermas, 1991; Webler,1995). As much as this concept found support amongmany analysts and regulators, its practical value has

been questioned in recent publications (Breyer, 1993;Coglianese, 1999; Rossi, 1997). Cary Coglianese men-tions six potential “pathologies” of consensus-seekingdeliberation (Coglianese, nd).

1. Tractability having priority over public impor-tance.

2. Regulatory imprecision.3. The lowest common denominator problem.4. Increased time and expense.5. Unrealistic expectations.6. New sources of conflict.

One of the major arguments has been that publicpreferences do not match the real interests of the pub-lic since the preferences are clouded by misinforma-tion, biases, and limited experience. Other argumentsagainst the use of deliberation in risk managementare that such processes are economically inefficient(Rosenbaum, 1978), that lay people are technicallyincompetent (Cross, 1998), that stakeholders are bi-ased and unable to represent the common interest(Coglianese, 1999; Reagan & Fedor-Thurman, 1987),and that deliberation incites conflict and further un-rest (Huntington, 1970). Hence, regulatory agenciesare confronted with a serious dilemma (Zeckhauser& Viscusi, 1996; Bohnenblust & Slovic, 1998): thedetermination of acceptable or tolerable environmen-tal impacts relies on subjective judgment and com-monly shared social values but there is no clear pro-cedure to amalgamate social concerns and values ina pluralist society and to find nonambiguous meansfor resolving value conflicts. Even if preferences andtradeoffs were known, a process is needed to in-tegrate divergent views, subjective rationalities, andpreferences into an effective setting for discussionabout compromise or consensus. In most cases, domi-nant (i.e., Pareto-optimal) options are not available sothat a compromise or a consensus must be based onbargaining, compensation, voting, or a legal verdict(Fischhoff, 1996). The question of whether more de-liberative procedures are able to resolve this prob-lem of ambiguity is contested in the literature (Chess,Dietz, & Shannon, 1998). However, any new approachto risk management should demonstrate how it in-cludes the resolution of ambiguities and conflictingvalues in its decision-making procedures.

2.6. Requirements for Regulatory Reform

Any attempt to develop a new approach for riskevaluation and management must be reviewed andtested against the five challenges mentioned above.


How does a new approach to risk management dealwith the constructivism-realism debate? How doesit incorporate physical consequences and social con-cerns? How does it deal with the different componentsof uncertainty? What are the normative inferencesdrawn from the resolution of the first and second de-bate and how are these inferences transformed intoappropriate risk management tools? What role candeliberation play in all of this?

3. A NOVEL APPROACH TO RISK DECISIONMAKING: RISK EVALUATION AND RISKCLASSIFICATION

3.1. Realism Versus Constructivism: The DualNature of Risk

There is no doubt that the term “risk” refers to theexperience of something that people fear or regard asnegative. It is also clear that this fear is extended to anevent or a situation that has not yet occurred but couldoccur in the future. Obviously, there are different di-mensions of what people label as negative impact orharm. People might fear damage to their health, theirwealth, their well-being, their self-esteem, or be con-cerned about violations of their central beliefs andvalues, cultural convictions, social status, and prestige.Within the professional communities on risk, most an-alysts would agree that damage to human health andthe environment are at the fore of what we call riskanalysis and risk management.

However, even if one excludes illusive phenom-ena such as value violations or cultural taboos andconcentrates only on human health and environmen-tal quality, there are still ambiguities. Human healthand, particularly, environmental quality are difficultto appraise from a purely objectivist viewpoint. Psy-chosomatic impacts, such as the multichemical syn-drome or the sick-house syndrome, make sense onlywhen viewed as a combination of exposure and psy-chological anxieties (Renn, 1997). Even more so, theappraisal of environmental quality rests on the atti-tude of the observer. For example, are alien speciesa risk or an enrichment for a given ecosystem? Doesthe fertilization of fields through anthropogenic air-borne nitrogen increase the risks or reduce the risksfor agricultural plants?

These questions are closely associated with thedebate on constructivism versus realism. Are theendpoints of risk assessments constructions of hu-man minds or do they represent physical entities thatare independent of subjective judgments? Since risk

refers to a potential of “real” consequences, in ouropinion it is both a social construction and a repre-sentation of reality. We agree on this point with JimShort and Eugene Rosa who insist that risk cannot beconfined to perceptions and social constructions alone(Short, 1989; Rosa, 1998). Neither can it be reduced toobjective outcomes in terms of injuries, fatalities, andother types of losses. Both the cultural attributionsas well as the physically measurable outcomes formthe content of the term “risk.” How people select is-sues of concern and how they model likelihood mayindeed be a result of cultural conventions and rules.The threat of being affected by these consequences,however, is real in the sense that people might suf-fer or lose their lives if the risk manifests itself in anaccident or a release of hazardous material.

The dual nature of risk demands a dual strategyfor risk management. Public values and social con-cerns may act as the driving agents for identifyingthose topics for which risk assessments are judged nec-essary or desirable. The magnitude of risks, however,should reflect technical expertise as best as possible,since “real” victims are on stage. Following this dualapproach, setting priorities within risk managementwould imply that social or political forces determinethe criteria of judging tolerable levels of risk, wherebythe technical assessments are used as one importantinput among others to quantify the extent of potentialdamage in time and space.

Based on these considerations, we propose to en-rich the set of criteria used to characterize risks. Formaking risk evaluation and management consistentwith the best scientific knowledge and the most ap-propriate social values, we consider it to be justifiedand necessary that both physical criteria as well as so-cial concerns are integral parts of these evaluations(Fiorino, 1989). Our list of criteria includes physicalas well as social indicators. Such a distinction rests ontwo assumptions.

1. It is possible and necessary to distinguish phys-ical from social and psychological attributes ofrisk.

2. Both sets of criteria are important for evalu-ating and managing risks.

We believe these assumptions are reasonable andreflect the dual nature of risk. The physical elementsshould be measured independent of social and psy-chological criteria unless there is clear evidence ofa common link. The psychological and social criteriashould be treated as criteria in their own right and notbe regarded as modifiers of the physical consequences.


In addition, risk assessments should include thosephysical components of risk that generate social con-cern. If someone is worried about a risk becauseit is ubiquitous or persistent, this risk characteristicshould be included in the evaluation process. Havingincluded such a concern does not imply using the intu-itive assessments (or guesses) of lay persons to judgeany given risk on this criterion. On the contrary, thebest technical estimate is needed to give justice to thelegitimate concern of the citizens. Even the personwho might suggest a new criterion for risk evalua-tion would agree that experts are needed to applythis criterion and to measure or calculate the likelyimpacts of each decision option on this specific crite-rion. Our approach is based on the idea that criteriafor evaluating risks should be developed from the so-cial discourse about concerns, while the “objective”measurement should be performed by the most pro-fessional experts at hand. These measurements mayturn out to be wrong, uncertain, or ambiguous, butthey are on average still more reliable than pure intu-ition or common sense since no methodological rulesare available for judging the quality of intuition inadvance.

3.2. Inclusion of Public Concerns: AdditionalCriteria for Evaluation

Once we decided to include more than the clas-sic components—extent of damage and probabilityof occurrence—the question remains: Which otherphysical and social impact categories do we want toinclude and how can we justify our selection? Theidentification of public concerns is not a trivial task.Empirical research has shown that people tend toevaluate risks on a large set of evaluative criteria ofwhich only few may claim universal validity. The fol-lowing contextual variables of risk have been foundto affect the perceived seriousness of risks in varyingdegrees (Boholm, 1998; Sjoberg, 1999; Slovic, 1987,1992; Rohrmann & Renn, 2000).

� The expected number of perceived fatalities orlosses.

� The catastrophic potential.� Qualitative risk characteristic such as volun-

tariness, personal control, familiarity, dread,and others.

� Emotional associations with the risk (stigma).� Trust in regulatory agencies and risk-handling

institutions.

� Social and cultural beliefs associated with thecause of risk or the risk-handling actors.

If the need for including public concerns into riskevaluation is accepted, one should use the results ofthe existing perception studies as the major heuristicrule for selecting the relevant criteria. Since the listof relevant criteria is long and not identical for dif-ferent groups, selection poses a serious problem. TheGerman Government’s Advisory Council on GlobalChange (WBGU) has addressed this problem in its1998 Annual Report (WBGU, 2000). The Council or-ganized several expert surveys on risk criteria (includ-ing experts from the social sciences) and performeda meta-analysis of the major insights from risk per-ception studies. The Council also consulted the lit-erature on similar approaches in countries such asthe United Kingdom, Denmark, the Netherlands, andSwitzerland (cf. Piechowski, 1994; Beroggi et al., 1997;Hattis & Minkowitz, 1997; Hauptmanns, 1997;Lofstedt, 1997; Petringa, 1997; Poumadere & Mays,1997). They asked experts to provide special reportson this issue to the authors. Nine criteria were finallychosen to represent most of the experts’ and publicconcerns as the result of a long exercise of delibera-tion and investigations. The experts, for example, wereasked to characterize risks on those dimensions thatthey would use for substantiating a judgment on tol-erability. These dimensions were compared in com-mon discussion sessions and those distilled that ap-peared most influential for characterizing differentrisks. These criteria are listed in Table I.

The last category, “mobilization,” was the onlycriterion aimed at describing public response (or out-rage) that found approval by all experts. After theWBGU proposal had been reviewed and discussedby many additional experts and risk managers, we de-cided to unfold the compact “mobilization index” anddivide it into four major elements.

1. Inequity and injustice associated with the dis-tribution of risks and benefits over time, space,and social status (thus covering the criterionof equity).

2. Psychological stress and discomfort associatedwith the risk or the risk source (as measuredby psychometric scales).

3. Potential for social conflict and mobilization(degree of political or public pressure on riskregulatory agencies).

4. Spill-over effects that are likely to be expectedwhen highly symbolic losses have repercus-sions on other fields such as financial markets


Table I. Criteria for Evaluating Risks

Criteria Description

Extent of damage Adverse effects in natural units such as deaths, injuries, production losses etc.Probability of occurrence Estimate for the relative frequency of a discrete or continuous loss functionIncertitude Overall indicator for different uncertainty componentsUbiquity Defines the geographic dispersion of potential damages (intragenerational justice)Persistency Defines the temporal extension of potential damages (intergenerational justice)Reversibility Describes the possibility to restore the situation to the state before the damage occurred (possible

restorations are, e.g., reforestation and cleaning of water)Delay effect Characterizes a long time of latency between the initial event and the actual impact of damage; the time of

latency could be of physical, chemical, or biological natureViolation of equity Describes the discrepancy between those who enjoy the benefits and those who bear the risksPotential of mobilization Understood as violation of individual, social, or cultural interests and values generating social conflicts

and psychological reactions by individuals or groups who feel inflicted by the risk consequences; thesecould also result from perceived inequities in the distribution of risks and benefits

or loss of credibility in management insti-tutions (Kasperson et al., 1988; Renn et al.,1992).8

The social criteria measure the additional effectwith respect to psychological or social responses be-yond the expected effect from acknowledging the per-formance of each risk on the other physical criteria.A similar decomposition has been proposed by theUK government (Environment Agency, 1998; Pollardet al., 2000; Kemp & Crawford, 2000). This proposalincludes two main criteria and three subcriteria each.

� Anxiety, divided into dread, unfamiliarity, andnotoriety.

� Discontent, divided into unfairness, imposi-tion, and distrust.

We believe that the inclusion of social criteria intothe formal risk evaluation process is still in its infancyand needs more refinement. Several agencies are nowpreparing such an extended evaluation process.

Expanding the scope of criteria for risk evalua-tion poses a risk in itself. Are risk management institu-tions able and capable of handling a set of nine criteria(further decomposed into subcriteria) within the timeconstraints under which they must operate? Is it real-istic to expect risk managers to consider a larger set offormal criteria in addition to damage and probability?Our suggestion is to stick with all the criteria but tomake the decision-making protocol easier to perform.To make the assessments on multiple criteria opera-tional for risk managers, we distinguish, as practiced

8 These spill-over effects have been the main target of the theoryof social amplification of risk. This theory was developed by aresearch team at Clark University in the late 1980s.

in many countries, three categories for handling risks(see Fig. 1): the normal area, the intermediate area,and the intolerable area (Piechowski, 1994).

The normal area is characterized by little statisti-cal uncertainty, low catastrophic potential, small num-bers when the product of probability and damage istaken, low scores on the criteria of persistency andubiquity of risk consequences, and reversibility of riskconsequences; in other words, normal risks are char-acterized by low complexity and are well understoodby science and regulation. In this case, the classic riskformula—probability times damage—is more or lessidentical with the “objective” threat. For risks locatedin the normal area, we follow the advice of most deci-sion analysts, who recommend risk-risk comparisonsand risk-benefit analysis as the major tool for risk re-duction or control (National Research Council, 1982).Such an analysis should be based on a risk-neutral at-titude. Risk aversion does not seem prudent to applyto these risks.

The intermediate area and the intolerable areacause more problems because the risks touch areasthat go beyond ordinary dimensions. Within theseareas the reliability of assessment is low, the statis-tical uncertainty is high, the catastrophic potentialcan reach alarming dimensions, and systematic knowl-edge about the distribution of consequences is miss-ing. The risks may also generate global, irreversibledamages, which may accumulate during a long timeor mobilize or frighten the population. An unequivo-cal conclusion about the degree of validity associatedwith the scientific risk evaluation is hardly possible. Inthis case, the attitude of risk aversion is appropriatebecause the limits of human knowledge are reachedand the remaining uncertainties are hard to charac-terize, let alone quantify.


Fig. 1. Risk areas: normal, intermediate, and intolerable area.

Source: WBGU, 2000.

3.3. The Handling of Uncertainty: Distinctionof Five Components

Since the issue of uncertainty is one of the majortopics of debate in the risk community, we tried to im-prove our conceptual understanding of this term. Thefirst component of uncertainty is the identification andjustification of probabilities linked to specific adverseeffects or distribution of effects. The term “proba-bility of occurrence” is used in the risk sciences forsuch events for which we have data on past trends,information about cyclical events, logical inferencesfrom experiments or systematic observations (such asmodeling dose-response relationships), or even sim-ple beliefs based on personal or institutional experi-ence. These data sources form the building blocks forestimating the relative frequency of an adverse effectover time, space, or subjects (such as human beings,animals, or ecosystems) (IEC, 1993). Uncertainty ina broader sense includes more than just probabili-ties. To be more systematic on this complex topic, wesuggest the following decomposition, reflecting thebroader concept of uncertainty (von Hasselt, 2000).

� Variability. Observed or predicted variation ofindividual responses to an identical stimulusamong the individual targets within a relevant

population such as humans, animals, plants,landscapes, etc. In risk management, safety fac-tors have been used to cover this variability.

� Systematic and random measurement errors.Imprecision or imperfection of measurement,problems of drawing inferences from smallstatistical samples, extrapolation from animaldata, biosurveys, or other experimental dataonto humans, uncertainties of modeling, in-cluding the choice of functional relationshipsfor extrapolating from large to small doses; allof these usually expressed through statisticalconfidence intervals.

� Indeterminacy. Resulting from a genuinestochastic relationship between cause and ef-fect(s), apparently noncausal or noncyclicalrandom events, or badly understood nonlin-ear, chaotic relationships.

� Lack of knowledge. Resulting from igno-rance, from the deliberate definition of sys-tem boundaries and hence exclusion from ex-ternal influences, measurement impossibilities,and others.

These components of incertitude are often highly cor-related. In this case one component can serve as ageneral indicator for the others. In many instances,


however, the four components may produce quite dif-ferent results. During the deliberative process gener-ating the WGBU annual report on global risks, weexperienced a heated debate about the uncertaintyconnected with the application of genetic technolo-gies for agricultural purposes (WBGU, 2000). Onegroup of scientists assessed this risk as low and notassociated with high uncertainty, while others weremuch more skeptical about the potential impactson ecosystems, and particularly highlighted remain-ing uncertainties. The conflict was resolved when weasked the participating experts to distinguish betweenthe different components of uncertainty. Althoughthe first group had based their judgment on the firsttwo components of uncertainty, the second group hadconsidered the third and, particularly, the fourth com-ponent for deriving their final judgment. In the end,both groups insisted more or less on their originaljudgment but each side was now able to understandthe reasoning behind the arguments of the other side.

3.4. Risk Classification: Six Different Risk Classes

Before addressing the remaining two major issuesof risk evaluation and management outlined in Sec-tion 2, it is necessary to include an intermediary stepthat provides the necessary link between risk assess-ment and evaluation. Given the nine criteria and thenumerous subcriteria, a huge number of risk classescan be deducted theoretically. But a huge numberof cases would not be useful for the purpose of de-veloping a comprehensive risk classification and cor-responding management strategies. In reality, somecriteria are tightly coupled and other combinationsare only possible theoretically. Considering the taskof setting risk management strategies, risks with oneor several extreme qualities need special attention.So such similar risk phenomena are subsumed underone risk class in which they reach or exceed the sameextreme qualities.

Events of damages with a probability of almostone were excluded from our classification. High po-tentials of damages with a probability of nearly oneare clearly located in the intolerable area and there-fore unacceptable. By the same token, probabilityheading toward zero is harmless as long as the asso-ciated potential of damage is small. We also excludedfrom the analysis small-scale accidents (with limiteddamage potential for each case) that reach large num-bers of victims due to their ubiquitous use (such as caraccidents). Given these specifications and exceptions,our exercise produced six different risk clusters thatwe have illustrated with Greek mythology (Klinke &

Renn, 1999). The mythological names were not se-lected for illustrative purposes only. When studyingthe Greek mythology of the time between 700 and 500BC, we became aware that these “stories” reflected thetransition from an economy of hunters and gatherersto an economy of agriculture and animal husbandry.This transition, with its dramatic changes, implieda new culture of anticipation and foresight. It alsomarked the transition from a human self-reflectionas being an object of nature to becoming a subjectof nature. The various mythological figures demon-strate the complex issues associated with the new self-awareness of creating the future rather than just beingat the mercy of fate.

3.4.1. Risk Class—Sword of Damocles

According to Greek mythology, Damocles wasonce invited by his king to a banquet. However, atthe table he had to eat his meal under a razor-sharpsword hanging on a fine thread. So chance and riskare tightly linked for Damocles and the Sword ofDamocles became a symbol for a threatening dangerin luck. The myth does not tell about a snapping of thethread with its fatal consequences. The threat insteadcomes from the possibility that a fatal event could oc-cur for Damocles any time even if the probability islow. This can be transferred to risks with large damagepotentials. Many sources of technological risks have avery high disaster potential, although the probabilitythat this potential manifests as a damage is extremelylow. So the prime characteristics of this risk class areits combination of low probability with high extentof damage. Typical examples are technological riskssuch as nuclear energy, large-scale chemical facilities,and dams. Beside the technological risks, natural haz-ards such as periodic floods, for example the 100-yearfloods, and meteorite impacts can be subsumed underthis category.

3.4.2. Risk Class—Cyclops

The ancient Greeks tell of mighty giants who werepunished by having only a single eye, the reason theywere called Cyclops. With only one eye, only one sideof reality can be perceived and the dimensional per-spective is lost. When viewing risks, only one sidecan be ascertained while the other remains uncertain.Likewise, for risks belonging to the class of Cyclopsthe probability of occurrence is largely uncertain,whereas the disaster potential is high and relativelywell known. A number of natural hazards, such asearthquakes, volcanic eruptions, nonperiodic floods,


and El Nino, belong to this category. There is oftentoo little knowledge about causal factors. In othercases, human behavior influences the probability ofoccurrence so that this criterion becomes uncertain.Therefore, the appearance of AIDS and other infec-tious diseases, as well as nuclear early warning systemsand NBC weapons,9 also belong to this risk class.

3.4.3. Risk Class—Pythia

The ancient Greeks consulted one of their ora-cles in cases of doubt and uncertainty. The most fa-mous was the Oracle of Delphi with the blind seeressPythia. Pythia intoxicated herself with gases in orderto make predictions and give advice for the future.However, Pythia’s prophecies were always ambigu-ous. Transferred to risk evaluation, that means thatboth the probability of occurrence as well as the ex-tent of damage remain uncertain. So the incertitude ishigh. This class includes risks associated with the pos-sibility of sudden nonlinear climatic changes, such asthe risk of self-reinforcing global warming or of theinstability of the West Antarctic ice sheet, with farmore disastrous consequences than those of gradualclimate change. It further includes technological risksas far-reaching innovations in certain applications ofgenetic engineering in agriculture and food produc-tion, for which neither the maximum amount of dam-age nor the probability of certain damaging events canbe estimated at the present. Finally, the Pythia classincludes chemical or biological substances for whichcertain effects are suspected, but neither their magni-tude nor their probability can be ascertained with anyaccuracy. The BSE risk is the best example of this.

3.4.4. Risk Class—Pandora’s Box

The old Greeks explained many hazards with themyth of Pandora’s box. This box was brought downto earth by the beautiful Pandora, who was createdby the god Zeus. Unfortunately, in addition to hope,the box contained many evils and scourges. As longas the evils and scourges stayed in the box, no dam-age at all had to be feared. However, when the boxwas open, all evils and complaints were released andcaused irreversible, persistent, and wide-ranging dam-age. A number of human interventions in the envi-

9 A study of the Peace Research Institute in Frankfurt, Germany,indicated that the Russian early warning system and the asso-ciated nuclear forces have considerable functional and mainte-nance deficiencies because of human behavior. See Muller andFrank (1997).

ronment also cause wide-ranging, persistent, and ir-reversible changes without a clear attribution to spe-cific damages—at least during the time of diffusion.Often, these damages are discovered only after theubiquitous diffusion has occurred. A good exampleis the effect of persistent organic pollutants (POPs).For example, CFCs are the main cause of the hole inthe ozone layer. One could also subsume under thiscategory effects of persistent chemicals on reproduc-tive functions, for example, endocrine disruptors. An-other example is changes in the ecosystem that remainstable over long periods. Here, particular attentionneeds to be given to risks characterized simultane-ously by high ubiquity, persistency, and irreversibility.Concerning the probability of occurrence and the ex-tent of damage only reasonable hypotheses are avail-able, so these criteria remain not only uncertain asis typical for Pythia risks, but the causal relationshipbetween agent and consequences is also not yet sci-entifically proven plausible.

3.4.5. Risk Class—Cassandra

Cassandra, a seeress of the Trojans, predicted cor-rectly the perils of a Greek victory, but her compatri-ots did not take her seriously. The risk class Cassandradwells on this paradox: the probability of occurrenceas well as the extent of damage are high and rela-tively well known, but there is a considerable delaybetween the triggering event and the occurrence ofdamage. That leads to the situation that such risks areignored or downplayed. The anthropogenic climatechange and the loss of biological diversity (WBGU,2001) are such risk phenomena. Many types of dam-age occur with high probability, but the delay effectleads to the situation that no one is willing to acknowl-edge the threat. Of course, risks of the Cassandra typeare only interesting if the potential of damage and theprobability of occurrence are relatively high. That iswhy this class is located in the “intolerable” area ofFig. 2.

3.4.6. Risk Class—Medusa

The mythological world of the ancient Greek wasfull of dangers that threatened people, heroes, andeven Olympic gods. The imaginary Gorgons were par-ticularly terrible. Medusa was one of the three imag-inary Gorgon sisters, feared because her appearanceturns the beholder to stone. Similar to the Gorgons,who spread fear and horror, some new phenomenahave a similar effect on modern people. Some innova-tions are rejected, despite the fact that they are hardly


Fig. 2. Risk classes.

Source: WBGU, 2000.

assessed scientifically as a threat, because they havespecial characteristics that make them individually orsocially frightening or unwelcome. Such phenomenahave a high potential of psychological distress and so-cial mobilization in public. This risk class is only ofinterest if there is a particularly large gap betweenlay risk perceptions and expert risk analysis. A typi-cal example is electromagnetic fields, whose extent ofdamage was assessed as low by most experts becauseneither epidemiologically nor toxicologically signif-icant adverse effects could be proven (Wiedemann,Mertens, & Schutz, 2000). Exposure, however, is wideand many people feel involuntarily affected by thisrisk.

3.5. The Use of the Classification in RiskEvaluation: A Decision Tree

To evaluate risks and set risk reduction priorities,we propose a procedure assigning risk potentials toone of the six risk prototypes of the classification. For

this purpose we use a decision tree in which five cen-tral questions must be answered (see Fig. 3).

3.5.1. First Question: Do We Have Some Knowledgeabout the Major Characteristics of the Risks?

If knowledge is not available on any of the cri-teria mentioned above, such unknown risks cannotbe handled as if they were known. Nonetheless, theymight have major importance because they are usu-ally associated with desirable innovations. Therefore,the aim must be to create institutional mechanismsthat provide almost automatic risk management re-sponses once the scope of potential impacts becomesvisible or detectable. The most important task here isto ensure that more knowledge about the unknownrisk potential is generated. This requires three ba-sic management strategies: the first one is to do a“quick and dirty” screening on the risk by means ofanalogy and comparison with similar situations; thesecond one is to provide sufficient public money for


Damagepotentialknown?

Disasterpotential

high?

Damage potentialand probabilityare uncertain

Other criteriahigh?

Pythia

Cassandra

no

yes

yes

Pandora

Mobilization is high Medusa

no

no

Probability is uncertain

Probability is low Damocles

Cyclops

Probability and delay effect are high

Ubiquity,persistency,irreversibility

are high

3. Quest.

4. Quest.

Improvingprecaution

Probabilityunknown, low,

or high?

yesReducing

disaster potential

Providingsubstitutions

Buildingconfidence

Ascertainingprobability

Strenghteningresponsibility

Thresholdson criteriaexceeded?

Assignment to the risk classes

no

Is therisk potential

known?

Eliminating major uncertainties,i.e., producing knowledge

Risk decision makingby means of

risk criteria and risk classes

yes

yes

no

Normal risks:routine management

2. Quest.

1. Quest.

Socialmobilization

high?

yes

no

5. Quest.

Fig. 3. Decision tree for evaluating and classifying risks.


investigating unknown risk potentials through furtherresearch; the third one is to make those who takethe risks or impose the risk on others liable for anydamage that may occur (by means of insurance, forexample). Insurance premiums then act as incentivesfor the risk originators to generate more knowledgeabout the potential impacts in due time. If knowledgeis already available anywhere in society, institutionalarrangements need to be in place to ensure that it isdisseminated to the affected parties and to politicaldecisionmakers (Zimmerman & Pahl, 1999).

3.5.2. Second Question: Does the Risk ExceedPrespecified Thresholds of One or MoreCriteria for Risk Characterization?

Each risk can be classified with more or less reli-ability on each of the nine criteria. The classificationitself has no impact on regulation unless interventionlevels are defined. These thresholds for action cannotbe determined in abstract. It may be one of the maintasks of the screening and evaluating regulatory bodyto define thresholds for action based on the nine cri-teria. Each risk candidate can then be tested againstthe predefined threshold. If a risk falls below any oneof the predefined thresholds, the risk potential canbe judged as “normal,” so that the existing structuresof routine management and regulation are sufficientto cope with them. Such “normal” risks are charac-terized by low complexity and are well understood byscience and risk decisionmakers. We locate these risksin the normal area (see Fig. 1).

If the risk potential exceeds any one of the thresh-olds on the evaluation criteria, it causes more prob-lems because the risks touch areas that go beyond or-dinary dimensions. In this case an assignment to oneof the risk classes is inevitable. For this purpose thenext question should be answered.

3.5.3. Third Question: Is the Damage PotentialKnown and Can it be Identified?

If the damage potential is unknown and cannot beidentified by risk experts, this criterion is associatedwith high uncertainty and entails that the probabilityof occurrence is also uncertain. As a result, the overallincertitude is high. This leads to the risk classes Pythiaor Pandora’s box. The Pythia risk class presumes thatthe scientific appraisal provides sufficient proof fora causal relationship between agent and effect butthat neither the quantitative extent of damage nor theprobabilities can be specified. Risks within the classof Pandora are those where only credible assumptions

about such a causal connection exist but no substan-tial evidence. Normally, such a risk would not fall intothe intermediate or intolerable area. However, if therisks are classified as being ubiquitous, persistent, andirreversible, a more cautious approach is warranted.In such a case, the risks cannot be significantly re-duced or even avoided if the worst case occurred andthe suspicion about negative impacts became true.If the damage potential is known and can be iden-tified, the next question will be relevant.

3.5.4. Fourth Question: Does the Assessed DamagePotential Exceed the Predefined Threshold forCatastrophic Potential?

If the experts assess the catastrophic potential asbeing high but the probabilities are either rated as lowor as unknown, one of the two risk classes Cyclops orSword of Damocles fits the description. The Cyclopsrisk class is characterized by a high extent of damage,whereas the probability of occurrence remains uncer-tain. The Damocles risk class is also characterized bya high disaster potential, but the probability that thispotential manifests itself as concrete damage is low,sometimes even minimal.

If both the disaster potential and the probabil-ity of occurrence are high, one would normally rejectsuch risks. Most likely, existing legal statutes wouldalready prohibit their occurrence. But if there is rele-vant delay between the triggering event and the dam-age impact, it leads to the situation that such risks areoften ignored and no one is willing to acknowledgethe threat. Such risks are characterized by the riskclass Cassandra.

3.5.5. Fifth Question: Does the Risk Show NoSignificantly High Values on Any PhysicalCriteria But May Load High on the SocialCriteria?

If the damage potential, the probability of occur-rence, the incertitude, and other physical criteria areassessed as low, such risks are usually not significantfor risk decisions. The exception is if the risk poten-tial triggers a high anxiety among individuals, violatesequity values, and/or produces a high potential of so-cial mobilization in public. In this case the damagepotential as well as the probability of occurrence arewidely known among scientists, i.e., any effect is be-low statistical significance level, although the effectmust not be zero. The hazardous nature of such risksis mainly based on the subjective perception of af-fected people that can lead to distress, anxiety, and,


sometimes, even to psychosomatic malfunctions. Peo-ple may also feel unequally treated, i.e., they perceivea discrepancy between those who are able to take ad-vantage of the benefits and those who bear the risks.These risk phenomena are subsumed under the riskclass Medusa since they generate a high mobilizationin the public. Once a risk is classified as producing ahigh mobilization potential, it is necessary to look intothe three components of the social mobilization crite-rion in order to design the appropriate managementand communication strategy.

4. FROM RISK EVALUATIONTO MANAGEMENT: RISK-BASED,PRECAUTIONARY, AND DISCURSIVESTRATEGIES

This section returns us to the five issues thatwe addressed in Section 2. The first three issues—“realism versus constructivism,” “inclusion of socialconcerns and perceptions,” and “the treatment of thedifferent components of uncertainty”—were locatedat the borderline between assessment and evalua-tion. This is why we treated those three issues underthe heading of risk evaluation. The next two issues—“choice of strategy” and “deliberation versus expertjudgments”—address topics that lie at the core of riskmanagement. Consequently these topics will be con-sidered in this section.

The essential objective of the proposed risk classi-fication is to derive effective, efficient, and politicallyand legally feasible strategies and measures for risk re-duction and mitigation. The characterization of eachrisk according to our proposal provides a knowledgebase for risk managers to select the most appropri-ate risk management strategies. Regardless of whichstrategy they may choose, the ultimate goal of eachstrategy is to transform unacceptable into acceptablerisks. The desired result of risk management effortis not to reduce all risks to zero but to move theminto the normal area, in which routine risk manage-ment and cost-benefit analysis becomes sufficient toensure overall safety and integrity (WBGU, 2000:18–20). By moving the risks into the normal area, theyoften pass through intermediary stages in which addi-tional strategies are needed in order to reach the finaldestination.

4.1. The Three Challenges: Complexity,Uncertainty, and Ambiguity

Similar to the classification of risk classes, it makessense to design a classification of generic risk manage-

ment strategies. These strategies focus on three majorchallenges that characterize the handling of risk issuesin society: complexity, uncertainty. and ambiguity (formore detail, see Renn, 1999c).

Complexity refers to the difficulty of identify-ing and quantifying causal links between a multitudeof potential candidates and specific adverse effects(cf. WBGU, 2000; Schellnhuber, 1999). The nature ofthis difficulty may be traced back to interactive ef-fects among these candidates (synergisms and antag-onisms) (National Research Council, 1988), positiveand negative feedback loops, long delay periods be-tween cause and effect, interindividual variation, in-tervening variables, and other effects. It is preciselythese complexities that make sophisticated scientificinvestigations necessary because the dose-effect re-lationship is neither obvious nor directly observable.Nonlinear response functions may also result fromfeedback loops that constitute a complex web of in-tervening variables.

The second problem refers to uncertainty. Uncer-tainty is different from complexity. As stated earlier, itcomprises different and distinct components such asstatistical variation, measurement errors, ignorance,and indeterminacy, which all have one feature in com-mon: uncertainty reduces the strength of confidencein the estimated cause and effect chain (for a similardefinition, see Priddat, 1996). If complexity cannot beresolved by scientific methods, uncertainty increases.But even simple relationships may be associated withhigh uncertainty if either the knowledge base is miss-ing or the effect is stochastic by its own nature.

The last term in this context is ambiguity orambivalence. This term denotes the variability of (le-gitimate) interpretations based on identical observa-tions or data assessments. Most of the scientific dis-putes in the fields of risk analysis and managementdo not refer to differences in methodology, measure-ments, or dose-response functions, but to the ques-tion of what all this means for human health and en-vironmental protection (Harrison & Hoberg, 1994).Emission data is hardly disputed. Most experts de-bate, however, whether an emission of x constitutes aserious threat to the environment or to human health.Ambiguity may come from differences in interpret-ing factual statements about the world or from differ-ences in applying normative rules to evaluate a stateof the world. In both cases, ambiguity exists on theground of differences in criteria or norms to interpretor judge a given situation. One example for the firstkind of ambiguity is pesticide residues in food wheremost analysts agree that the risk to human healthis extremely low yet many demand strict regulatory


actions. A classic example of the second kind is smok-ing: Should the government regulate smoking becausethe link to cancer is clearly confirmed or should it re-frain from regulation since people voluntarily acceptthis risk (not including passive smoking, for the sakeof the argument)? If we look at the practice in differ-ent countries, we can observe a variety of approachesto deal with smoking, ranging from strict to almost noregulatory actions. High complexity and uncertaintyfavor the emergence of ambiguity, but there are alsoquite a few simple and almost certain risks that cancause controversy and thus ambiguity. It is thereforeimportant to distinguish between complexity, uncer-tainty, and ambiguity: these three terms are correlatedbut they are not identical.

4.2. Managing Complexity, Uncertainty,and Ambiguity

Different evaluation and management strategiesfollow from the analysis of these three challenges. Ifthe problem is complexity, a risk manager is well ad-vised to bring the best expertise together and regulateon the basis of the state-of-the-art knowledge in riskassessment. It does not make much sense to incorpo-rate public concerns, perceptions, or any other socialaspects into the function of resolving (cognitive) com-plexity (for a similar argument, see Charnley, 2000)unless specific knowledge of these groups help to un-tangle complexity (Charnley, 2000:16f). Complex phe-nomena demand almost equally complex methods ofassessments. These methods can be offered by scien-tists and experts better than by anybody else. In termsof regulatory actions, quantitative safety goals andconsistent use of cost-effectiveness methods are theappropriate tools to deal with complex risk problemsthat show little uncertainty and no ambiguity.

If the problem is uncertainty, however, knowl-edge is either not available or unattainable due tothe nature of the hazard. Under these circumstances,risk managers have to rely on resilience as the guid-ing principle for action (WBGU, 2000:288–292). Mostof the precautionary management tools would fallinto this category. Knowledge acquisition may helpreduce uncertainty and thus move the risk back tothe first stage of handling complexity. If uncertaintycannot be reduced by additional knowledge, how-ever, or if action is demanded before the necessaryknowledge can be obtained, the routine managementstrategies for resolving complexity would be incom-plete because the objective here is to act prudentlyunder the condition of uncertainty. Acting prudently

means to design resilient measures that allow flexibleresponses to unexpected events (surprises) (Stirling,1998; WBGU, 2000:289ff). Management tools thatwould fit the resilience approach include containmentin space and time (to make exposure reversible), con-stant monitoring, development of equi-functional re-placements, and investments in diversity and flexibil-ity (Collingridge, 1996; Klinke & Renn, 2001). Classicregulatory strategies such as the ALARA principle(as low as reasonably achievable), BACT (best avail-able control technology), or state of technology arealso elements of this approach (WBGU, 2000:217–218).

Decisions based on uncertainty management re-quire, therefore, more than input from risk specialists.They need to include stakeholder concerns, economicbudgeting, and social evaluations. The focal point hereis to find the adequate and fair balance between thecosts of being overcautious versus the costs of be-ing not cautious enough (van den Daele, 2000). Sinceboth costs are almost impossible to quantify due tothe remaining uncertainties, subjective judgments areinevitable. This means that the setting of painful valuetradeoffs is inevitable. The trade-off ratios determinewho will bear the costs—either in form of additionaldamages by being not cautious enough or in form ofregulatory costs for being overcautious. It is obviousthat those who bear either of the two costs are entitledto be the main negotiators for setting the necessarytradeoffs.

Setting tradeoffs is even more complex when itcomes to resolving ambiguity. Although scientific ex-pertise is essential for gaining a better understandingof ambiguities and dealing with them in an enlight-ened manner, it cannot prescribe the value tradeoffsnecessary to cope with the ambiguities (Yankelovich,1991). In addition, ambiguities cannot be resolved byincreased efficiency since the outcome in itself is con-troversial not just the distribution of costs.

Genetically modified organisms for agriculturalpurposes may serve as an example here. Our own sur-veys on the subject demonstrate that people, for socialand moral reasons, associate high risks with the appli-cation of gene technology (Hampel & Renn, 2000).Whether the benefits to the economy balance the coststo society in terms of increased health risks is not amajor concern of the polled public. People disagreeabout the social need for genetically modified foodin Western economies where abundance of conven-tional food is prevalent. They are worried about theloss of personal agency when selecting and preparingfood, about the long-term impacts of industrialized


Table II. Risk Management Challenges and Corresponding Strategies

Challenge Objective Function Strategies Instruments

Complexity Effective, efficient, Agreement on causal Reducing Standards, risk-riskand adequate relations and effective measures damage potential; comparisons,protection limiting over-all risk level cost-effectiveness,

risk-benefit analysisUncertainty Resilience Avoiding irreversibilities Diversity, Containment in time

and vulnerabilities flexibility limiting range of and space, development ofeffects substitutes

Efficient and fair Balancing costs of Trade-off Negotiated rule making,distribution of underprotection analysis decision analysisburden(s) vs. costs of overprotection

facing uncertain outcomesAmbiguity Socially Resolving value conflicts and Consensus Advisory commitees, citizen

acceptable assuring fair treatment of concerns seeking discourse panels, value-tree analysisdevelopment and visionspath

agriculture, and the moral implications of tamperingwith nature.10 These concerns cannot be addressedby either scientific risk assessments or by determiningthe right balance between over- and underprotection.The risk issues in this debate focus on differences ofvisions about the future, basic values and convictions,and the degree of confidence in human ability to con-trol and direct its own technological destiny. Thesewider concerns require the inclusion of those whoexpress or represent them. To make these concernsoperational for risk management, some kind of dis-course for conflict resolution and joint vision buildingis required. Coping with ambiguity necessarily leadsto discursive management tools, i.e., communicativeprocesses that promote rational value disputes (Rippe& Schaber, 1999). This is the place where delibera-tive processes are required from a social-analytical aswell as normative viewpoint. The ultimate goal of riskregulation in the face of ambiguities is a consensusor a compromise between those who believe the riskis worth taking (maybe because of self-interest) andthose who believe that the pending consequences donot justify the potential benefits of the risky activity ortechnology.

Table II provides a summary of the three manage-ment styles and their basic strategies and instruments.Complex risks require sophisticated methods for as-sessing and regulating risks. Conflicts arise as a result

10 The importance of the variable “tampering with nature” wasbrought to our attention by Lennard Sjoberg of the SwedishSchool of Economics (Sjoberg, 1999).

of cognitive disputes over models and rationales forselecting effective as well as efficient risk reductionmeasures. Dealing with uncertainty involves two ob-jectives: providing resilient strategies to be preparedfor surprises and finding an adequate and fair balancebetween assumed over- and underprotection. Ambi-guities reflect value conflicts, which need to be recon-ciled in consensus seeking exercises.

Most risks are characterized by a mixture of com-plexity, uncertainty, and ambiguity. Smoking may bea good example for low complexity and uncertaintybut high ambiguity. Nuclear energy may be a goodcandidate for high complexity and high ambiguitybut relatively little uncertainty. Endocrine disrupterscould be cited as examples for high complexity, un-certainty, and ambiguity. We could continue the listforever. When looking at our own classification, how-ever, we can bring some more order into this webof interrelationships. The two risk classes Damoclesand Cyclops are characterized by high complexity andhence require mainly risk-based management strate-gies, the risk classes Pythia and Pandora are charac-terized by high uncertainty and hence demand theapplication of precautionary strategies, and the riskclasses Cassandra and Medusa load high on ambigu-ity and thus require discursive management strate-gies for building trustworthiness, social consent, andpublic confidence. This distinction does not mean thatwithin each risk class the other strategies and instru-ments have no place, but they do take a “back seat.”The following section describes the appropriate riskmanagement tools for each of the three managementstrategies.


4.3. The Three Management Styles: Risk-Based,Precaution-Based, Discourse-Based

4.3.1. Risk-Based Management

The risk classes Damocles and Cyclops requirethe application of risk-based strategies and regula-tion. Nuclear energy, large chemical facilities, dams,nuclear early warning systems, and NBC weapons areobvious examples. However, the appearance of well-known infectious diseases is also a representative ofthese risk classes. Within the risk class Damocles, boththe probability of occurrence and extent of damageare relatively well known. Since the damage compo-nent is the one that triggers concern, risk managersshould concentrate their efforts on reducing the dis-aster potential. For example, in the past, the primarystrategy of nuclear energy was to reduce the proba-bility of a core meltdown. More useful would havebeen a change toward reducing the catastrophic po-tential (meanwhile this strategy has been pursued inthe development of new reactor types).

Within the risk class Cyclops, a mixture of risk-based and precautionary strategies is useful becausethe risk potentials are characterized by good knowl-edge on the extent of damage, but the distributionof probabilities is relatively unknown. To remedy thisdeficit, increasing research and thorough monitoringfor specifying the distribution of probabilities is re-quired. Strict liability and compulsory insurance forthose generating the risks may provide an additionalincentive to reduce the disaster potential and to pre-vent unwelcome surprises: operators are encouragedto improve their knowledge and to decrease the re-maining uncertainties.

Several tools from uncertainty management arealso recommended for risks with high disasterpotential. Capacity building improves the institu-tional and organizational structures and guaranteescontrol over procedures of licensing, monitoring,training, and so forth. Additionally, technical pro-cedures such as redundancy, organizational securityunits, the integration of latitudes, buffers, elasticities,and diversification, that is, the local distribution of risksources, can decrease vulnerabilities.

4.3.2. Precaution-Based Management

The risk classes Pythia and Pandora fall intothis management category. Typical examples of theserisk classes are the release of transgenic plants, spe-cific applications of genetic engineering, the increas-ing greenhouse effect, persistent organic pollutants

(POP), and endocrine disruptors. These risk poten-tials are characterized by a relatively high degree ofuncertainty. Therefore, the first priority of risk man-agement must be the application of precautionarymeasures and the development of substitutes. Man-agement tools include:

� Containment of application in space and time.� Constant monitoring of potential side effects.� Development of functional equivalents with

less persistent or ubiquitous consequences.� Promoting diversity and flexibility.� Capacity building for organizational compe-

tence.� Building high-reliability organizations for han-

dling uncertain risks.� Introduction of strict liability.� Classic tools such as ALARA, BACT, etc.

The improvement of knowledge constitutes animportant element of dealing with those risks becausefurther knowledge may reduce the remaining uncer-tainties. An ideal program softens the precautionarymeasures in line with the additional knowledge gen-erated through research and by containing the distri-bution of the risk over time and space, i.e., limitingthe scope of potentially irreversible damages.

4.3.3. Discourse-Based Management

The third category, with discursive strategies, isessential if either the potential for wide-ranging dam-age is ignored, due to delayed effect such as climatechange, or—the opposite—harmless effects are per-ceived as threats, an example being electromagneticfields. The risk classes Cassandra and Medusa repre-sent these risks. They are not associated with muchscientific uncertainty or complexity. In the case ofCassandra, human beings do not take the risks se-riously because of the lingering delay between theinitial event and the damage impact. Within the riskclass Medusa, the probability of occurrence and theextent of damage are relatively well known, i.e., theassessment quality for characterizing the risks is atleast satisfactory. The hazardous nature of the risk ismainly based on the subjective perception. The beliefthat the risk poses a serious threat produces publicattention and fuels public anxiety, which leads to dis-tress, psychosomatic malfunctions, or social outrage.

Therefore, these risk classes require strate-gies building up consciousness, building confidence,strengthening trustworthiness in regulatory bodies,and initiating collective efforts of institutions for


taking responsibility. These are social goals that can-not be accomplished by risk experts or regulatorsalone. If ambiguity governs the risk debate, oneneeds discursive methods of deliberation and deci-sion making—clarification of facts in such discoursesis not enough and will not convince people that therisks belong in the normal area. What is needed is theinvolvement of affected people so that they are ableto integrate the remaining uncertainties and ambigu-ities into their own procedures of assigning tradeoffs.Ambiguities demand public participation.

5. IMPLICATIONS FOR ANALYSISAND DELIBERATION

The juxtaposition of the three management stylescould lead to the misperception that deliberation andpublic involvement are only necessary when ambigu-ities occur. Indeed, we claim that deliberation is notthe most essential element for reducing complexity oruncertainty, but that it may be helpful even for risksthat do not pose major ambiguities. As much as scien-tific input about the physical risk parameters providesa mandatory component of deliberative actions aimedat resolving ambiguities, so do procedures for reduc-ing complexity and uncertainty require deliberativeelements.

First, resolving complexity requires deliberationamong experts. We have given this type of deliber-ation the title “epistemiological discourse.”11 Withinan epistemiological discourse, experts (not necessar-ily scientists) argue over the factual assessment withrespect to the criteria that we have proposed and de-scribed above. The objective of such a discourse is themost adequate description or explanation of a phe-nomenon (for example, the question of which physicalimpacts are to be expected by the emission of specificsubstances). The more complex, the more multidis-ciplinary, and the more uncertain a phenomenon ap-pears to be, the more necessary is a communicativeexchange of arguments among experts. The goal is toachieve a homogeneous and consistent definition andexplanation of the phenomenon in question as well asa clarification of dissenting views. The discourse pro-duces a profile of the risk in question on the selectedcriteria.

Second, an epistemiological discourse may revealthat there is more uncertainty and ambiguity hiddenin the case than the initial appraisers had anticipated.

11 The following classification of discourse types was first publishedin Renn (1999a).

If the cognitive discourse includes natural as well associal scientists, future controversies and risk debatesmay be anticipated. Risk controversies would be lesssurprising. Since our criteria include aspects of per-ception and social mobilization, these assessmentsmay serve as early warning systems for controversiesto come.

Epistemiological discourses can be organized indifferent forms. One popular example is the con-sensus conference, a method routinely applied forresolving cognitive conflicts in defining the most suit-able medical treatment. In our own institution (Cen-ter of Technology Assessment) we have used groupdelphi techniques and meta-analytical workshops asappropriate instruments for conducting discourses onclarifying knowledge (Webler et al., 1991; Wachlin &Renn, 1999).

If risks are associated with high uncertainty, sci-entific input is only the first step of a more complexevaluation procedure. It is still essential to compile therelevant data and the various arguments for the posi-tions of the different science camps. Procedures suchas the “Pedigree Scheme” by Funtowicz and Ravetz(different classes for judging the robustness of a the-ory or causal statement) might be helpful to organizeand classify existing knowledge (Funtowicz & Ravetz,1999). In a second step, information about the dif-ferent types of uncertainties must be collected andbrought into a deliberative arena. Players in this arenaare not only scientists, but also directly affected stake-holders and public interest groups (Yosie & Herbst,1998). The objective here is to find the right balancebetween too little and too much precaution. Thereis no scientific answer to this question and even eco-nomic balancing procedures are of limited value sincethe stakes are uncertain. We have coined this type ofdeliberation “reflective discourse.”

Reflective discourse deals with the clarification ofknowledge (similar to the cognitive) and the assess-ment of tradeoffs between the competing extremesof over- and underprotection. Reflective discoursesare mainly appropriate as a means to decide on risk-averse or risk-prone approaches to innovations. Thisdiscourse provides answers to the question of howmuch uncertainty one is willing to accept for somefuture opportunity. Is taking the risk worth the po-tential benefit? The classic question of “how safe issafe enough” is also an integral part of this type ofdiscourse. We would recommend that policymak-ers, representatives of major stakeholder groups, andscientists take part in reflective discourses. Politicalor economic advising committees that propose or


evaluate political options could also be establishedto advise this core group. Special procedures such asnegotiated rule making, mediation, and roundtablesare most appropriate for reaching the desired purposeof reflective discourses (Fiorino, 1995). As structur-ing instruments we have used value-tree analysis andother decision-aiding tools (Renn et al., 1993).

The last type of deliberation, which we have calledparticipatory discourse, is focused on resolving ambi-guities and differences about values. Established pro-cedures of legal decision making, but also novel pro-cedures, such as citizen advisory panels and citizenjuries, belong to this category. Participatory dis-courses are mainly appropriate as means to searchfor solutions that are compatible with the interestsand values of the people affected and to resolve con-

Simple Complex Uncertain Ambiguous

Discourse:internal

Discourse:reflective

Discourse:cognitive

Discourse:participatory

Actors:Agency Staff

Routine Operation

Scientific RiskAssessmentNecessary

Type of Conflict:cognitive

Actors:Agency Staff

External Experts

Risk BalancingNecessary

Risk TradeoffAnalysis andDeliberationNecessary

Actors:Agency Staff

External ExpertsStakeholders such as

Industry, DirectlyAffected Groups

Actors:Agency Staff

External ExpertsStakeholders such as

Industry, DirectlyAffected GroupsRepresentativesof the Public(s)


evaluative


evaluativenormative

Risk AssessmentNecessary

Risk AssessmentNecessary

Risk BalancingNecessary

Fig. 4. The risk management escalator (from simple via complex and uncertain to unambiguous phenomena).

flicts among them. This discourse involves subjec-tive weighting of the criteria and an interpretationof the results. Issues of fairness and environmentaljustice, visions on future technological developmentsand societal change, and preferences about desirablelifestyles and community life play a major role in thesedebates. In our Center, we have experimented withcitizen panels or juries (randomly selected), volun-tary advisory groups, the Danish model of lay per-sons’ consensus conferences, and other participatorytechniques in order to resolve ambiguities and valueconflicts (Schneider, Oppermann, & Renn, 1998).

Fig. 4 provides an overview of the different dis-course requirements when one moves from simpleto complex, from complex to uncertain and fur-ther to ambiguous risk issues. Although simple risks


require nothing more than routine actions by regula-tors, more complex and uncertain risk issues demandinput from external science communities and majorstakeholders. Broader citizen participation is neededfor dealing with ambiguities. In many cases, it may bedifficult to allocate a given risk into this scheme. Ob-viously, one needs a screening exercise to position therisk in accordance with our decision tree and to char-acterize the degree of complexity, uncertainty, andambiguity associated with the risk under investiga-tion. We would recommend a “risk characterizationpanel” consisting of experts (natural and social sci-entists), some major representatives of stakeholders,and regulators who will perform this initial screening(WBGU, 2000). Depending on the judgment of thispanel, risk information is then processed according tothe risk class in which the risk is located as a resultof the screening exercise and according to the typeof management deemed necessary to deal with com-plexity, uncertainty, and ambiguity.

It is clear that all three management strategiesneed to be combined and all three types of discoursesintegrated into one major deliberative process if risksfall in all three categories. Our experiences, however,have been that it is essential to distinguish the type ofdiscourse that is needed to resolve the issue at ques-tion. Knowledge questions such as the right extrapo-lation method for transferring animal data to humansshould not be resolved in a participatory discourse.Similarly, value conflicts should not be resolved inepistemiological discourse settings. It seems advisableto separate the treatment of complexity, uncertainty,and ambiguity in different discourse activities sincethey each need a different form of resolution. Often,they need different participants, too. We have madean attempt to provide a hybrid model of deliberationcalled the cooperative discourse model that combinesthe three discourse types into one connected activitywithout giving up the analytical separation betweenthe three parts (Renn, 1999b).

6. SUMMARY AND CONCLUSIONS

Our concept of nine risk evaluation criteria, sixrisk classes, a decision tree, and three managementcategories was developed to improve the effective-ness, efficiency, and political feasibility of risk man-agement procedures. The main task of risk evalua-tion and management is to develop adequate tools fordealing with the problems of complexity, uncertainty,and ambiguity. Based on the characteristics of differ-ent risk types and these three major problems, we dis-

tinguished three types of management—risk-based,precaution-based, and discourse-based strategies.

The risk-based strategy is the common solution torisk problems. Once the probabilities and their corre-sponding damage potentials are calculated, risk man-agers are required to set priorities according to theseverity of the risk, which may be operationalized asa linear combination of damage and probability oras a weighted combination thereof. Within our newrisk classification, the two central components havebeen augmented with other physical and social crite-ria that still demand risk-based strategies as long asuncertainty is low and ambiguity absent. Risk-basedstrategies are best solutions to problems of complex-ity and some components of uncertainty, for example,variation among individuals. If the two most impor-tant risk criteria, probability of occurrence and extentof damage, are relatively well known and little un-certainty is left, the traditional risk-based approachseems reasonable.

If uncertainty plays a large role, in particular,indeterminacy or lack of knowledge, the risk-basedapproach becomes counterproductive. Judging therelative severity of risks on the basis of uncertain pa-rameters does not make much sense. Under these cir-cumstances, management strategies belonging to theprecautionary management style are required. Theprecautionary approach has been the basis for muchof the European environmental and health protec-tion legislation and regulation. Our own approach torisk management has been guided by the propositionthat any conceptualization of the precautionary prin-ciple should be (1) in line with established methods ofscientific risk assessments, (2) consistent and discrim-inatory (avoiding arbitrary results) when it comes toprioritization, and (3) at the same time, specific withrespect to precautionary measures, such as ALARAor BACT, or the strategy of containing risks in timeand space. This suggestion does, however, entail a ma-jor problem: looking only to the uncertainties does notprovide risk managers with a clue about where to setpriorities for risk reduction. Risks vary in their de-gree of remaining uncertainties. How can one judgethe severity of a situation when the potential damageand its probability are unknown or contested? In thisdilemma, we advise risk managers to use additionalcriteria of hazardousness, such as “ubiquity,” “irre-versibility,” and “pervasiveness over time,” as proxiesfor judging severity.

Our approach also distinguishes clearly betweenuncertainty and ambiguity. Uncertainty refers to asituation of being unclear about factual statements;


ambiguity to a situation of contested views aboutthe desirability or severity of a given hazard. Uncer-tainty can be resolved in principle by more cognitiveadvances (with the exception of indeterminacy), am-biguity only by discourse. Discursive procedures in-clude legal deliberations as well as novel participa-tory approaches. In addition, discursive methods ofplanning and conflict resolution can be used. If am-biguities are associated with a risk problem, it is notenough to demonstrate that risk regulators are opento public concerns and address the issues that manypeople wish them to take care of. The process of riskevaluation itself needs to be open to public input andnew forms of deliberation. We have recommended atested set of deliberative processes that are, at least inprinciple, capable of resolving ambiguities in risk de-bates (for a review, see Renn, Webler, & Wiedemann,1995).

Deliberative processes are needed, however, forall three types of management. Risk-based manage-ment relies on epistemiological, uncertainty-basedmanagement on reflective, and discourse-based man-agement on participatory discourse forms. Thesethree types of discourse could be labeled as ananalytic-deliberative procedure for risk evaluationand management. We see the advantage of a delibera-tive style of regulation and management in a dynamicbalance between procedure and outcome. Procedureshould not have priority over the outcome; outcomeshould not have priority over the procedure. An intel-ligent combination of both can elaborate the requiredprerequisites of democratic deliberation and its sub-stantial outcomes to enhance the legitimacy of politi-cal decisions (Guttman & Thompson, 1996; Bohman,1997, 1998).

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