Tell me ‘how’ or ‘why’ –

A Heuristic to Choose between Qualitative

and Quantitative Systems Analysis

Alexander Zock1 and Andreas Größler2

Purpose:

Provide a heuristic to decide whether quantitative or qualitative system

dynamics projects should be conducted.

Motivation:

Knowing the answer to this question is a prerequisite for choosing the

right SD approach in a project and is thus at the start of a project

helpful for the planning of resources and expected outcomes.

Heuristic:

The heuristic is based on the observation that every SD project must

start with a clear problem articulation. It proposes to implement this

problem articulation step by the help of two guiding questions that if

found appropriate, define a given problem as either being of a

qualitative or a quantitative nature.

Based on this classification scheme the heuristic recommends the use

of qualitative mapping (QMAP) in the case of a qualitative problem and

the use of quantitative modeling (QMOD) in the case of a quantitative

problem. The heuristic avoids the “To map or to model”-question that

often complicates the process of using SD in a project, by mapping the

right problems to the right methodology instead of trying to support the

use of either approach in an “either or” fashion.

The guiding questions of the heuristic can be stated as follows:

• QMAP-question:

How are variables X1, X2, X3 ….. causally related?

A QMAP problem is usually a structural problem (focus is on

closed causal feedback loops)!

• QMOD-question:

Why has (have) variable(s) X (X1, X2, X3 ….) („reference mode“)

developed this way between times T1 and T2 („time horizon“)?

A QMOD-problem is usually a complex dynamical problem!

Caveat:

The heuristic does not work when the problem is not about a structure-

behavior relationship (e.g. when pure forecasting or a prognosis is the

aim).

1 European Center for Aviation Development, Darmstadt, Germany, 1Email: alexander.zock@ecad-aviation.de

2 Radboud University Nijmegen, Nijmegen, The Netherlands, 1Email: a.groessler@fm.ru.nl

• The question of whether a given SD project is trying to address a qualitative or a quantitative problem can be answered via the proposed heuristic.

• QMAP or QMOD are two modes of operation in a SD based project and are not in conflict to each other, but are the tools of choice for their respective types of problems.

• The answer to the old question “To map or to model” does not lie in a tool discussion but must be looked for in the problem perception of the person or group that defines the given SD problem.

“If you want to cut a block of wood use an axe. If you want to hammer a nail use a hammer. You can use both tools to hammer a nail, but not to cut a block of wood.”

So, a discussion about the experience you had while using both tools in different contexts does not really clarify the issue. Only when you start to think about your motivation for action will you have the chance to choose the right tool in a conscious way. QMAP and QMOD are two modes of observation.

QMOD- quantitative problem articulation

• Theme selection: What is the problem? Why is it a

problem?

• Key variables: What are the key variables we must

consider?

• Time Horizon: How far into the future should we

look? How far back in the past lie the roots of the

problem?

• Dynamic problem definition (Reference modes in

graphical display): What is the historical behavior of

the key variables on this time scale? What might their

behavior be in the future?

• Guiding Question: Why has (have) variable(s) X

(X1, X2, X3 ….) („reference mode“) developed this

way between times T1 and T2 („time horizon“)?

QMAP- qualitative problem articulation

• Theme selection: What is the problem? Why is it a

problem?

• Key variables: What are the key variables we must

consider?

• Causal Context: What is the context of the problem

we want to understand? What are the main context

variables?

• Structural problem definition: What is it that we are

trying to understand? Where is the causal uncertainty

that we want to explore?

• Guiding Question: How are variables X1, X2, X3 …..

causally related?

Case study 1:

Topic: Identification of the main cost drivers

in the sale department of an airline.

Guiding question: How are the main levers

of an airline sales department causally related

to the overall costs caused by the sales

department of an airline.

Result: CLD that resulted from a three hour

workshop with the project team in this Case

Study (QMAP-example).

Case Study 2:

Topic: Policy analysis for the cockpit planning

analysis in an airline.

Guiding question: Why does the

relationship between demand and supply for

cockpit personnel of an airline show a

“Bullwhip Effect” of the magnitude observed

in the last 15 years.

Result: SFD of the pilot aging chain as it

was documented in the framework of this

Case Study (QMOD-example).

A proposed problem articulation scheme:

Examples:

Conclusions: