The Place of Statistics in Psychology

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  • http://epm.sagepub.com/MeasurementPsychological

    Educational and

    http://epm.sagepub.com/content/20/4/641The online version of this article can be found at:

    DOI: 10.1177/001316446002000401

    1960 20: 641Educational and Psychological MeasurementJum Nunnally

    The Place of Statistics in Psychology

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  • 641

    THE PLACE OF STATISTICS IN PSYCHOLOGY

    JUM NUNNALLY

    University of Illinois

    EDUCATIONAL AND PSYCHOLOGICAL MEASUKEtfEKTVol. XX. No. 4. 1960

    MosT psychologists probably will agree that the emphasis onstatistical methods in psychology is a healthy sign. Although wesometimes substitute statistical elegance for good ideas and over-embellish small studies with elaborate analyses, we are probablyon a firmer basis than we were in the prestatistieal days. However,it will be argued that there are some serious m1iiemphases in ouruse of statistical methods, which are retarding the growth ofpsychology.

    The purpose of this article is to criticize the use of statistical&dquo;hypothesis-testing&dquo; models and some related concepts. It will beargued that the hypothesis-testing models have little to do withthe actual testing of hypotheses and that the use of the models hasencouraged some unhealthy attitudes toward research. Some al-ternative approaches will be suggested.Few, if any, of the criticisms which will be made were orig-

    inated by the author, and, taken separately, each is probably awell-smitten &dquo;straw man.&dquo; However, it is hoped that when thecriticisms are brought together they will argue persuasively for achange in viewpoint about statistical logic in psychology.

    What tg WrongMost will agree that science is mainly concerned with finding

    functional relations. A particular functional relationship may bestudied either because it is interesting in its own right or becauseit helps clarify a theory. The functional relations most often soughtin psychology are correlations between psychological variables, anddinerences in central tendency in differently treated groups of sub-

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  • 642 EDUCATIONAL AND PSYCHOLOGICAL MEASUREMENT

    jects. Saying it in a simpler manner, psychological results are us-ually reported as correlation coefficients (or some extension thereof,such as factor analysis) and differences between means (or someelaboration, such as a complex analysis of variance treatment).

    Hypothesis Testing. After an experiment is completed, and thecorrelations or differences between means have been obtained, theresults must be interpreted. The experimenter is aware of samplingerror and realizes that if the experiment is run on different groupsof subjects the obtained relations will probably not be the same.How then should he take into account the chance element in theobtained relationship? In order to interpret the results, the experi-menter would, as most of us have, rely on the statistical models forhypothesis testing. It will be argued that the hypothesis-testingmodels are inappropriate for nearly all psychological studies.

    Statistical hypothesis testing is a decision theory: you have oneor more alternative courses of action, and the theory leads to thechoice of one or several of these over the others. Although the theoryis very useful in some practical circumstances (such as in &dquo;qualitycontrol&dquo;), it is misnamed. It has very little to do with hypothesistesting in the way that hypotheses are tested in the work-a-dayworld of scientific activity.The most misused and misconceived hypothesis-testing model

    employed in psychology is referred to as the &dquo;null-hypothesis&dquo;model. Stating it crudely, one null hypothesis would be that twotreatments do not produce different mean effects in the long run.Using the obtained means and sample estimates of &dquo;population&dquo;variances, probability statements can be made about the acceptanceor rejection of the null hypothesis. Similar null hypotheses are ap-plied to correlations, complex experimental designs, factor-analyticresults, and most all experimental results.Although from a mathematical point of view the null-hypothesis

    models are internally neat, they share a crippling flaw: in the realworld the null hypothesis is almost never true, and it is usually non-sensical to perform an experiment with the sole aim of rejectingthe null hypothesis. This is a personal point of view, and it cannotbe proved directly. However, it is supported both by common senseand by practical experience. The common-sense argument is thatdifferent psychological treatments will almost always (in the longrun) produce differences in mean effects, even though the differences

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  • 643JUM NUNNALLY

    may be very small. Alsof just as nature abhors a vacuum, it proba-bly abhors zero correlations between variables.

    Experience shows that when large numbers of subjects are usedin studies, nearly all comparisons of means are &dquo;significantly&dquo; dif-ferent and all correlations are &dquo;significantly&dquo; different from zero.The author once had occasion to use 700 subjects in a study of pub-lie opinion. After a factor analysis of the results, the factors werecorrelated with individual-difference variables such as amount of

    education, age, income, sex, and others. In looking at the resultsI was happy to find so many &dquo;significant&dquo; correlations (under thenull-hypothesis model)-indeed, nearly all correlations were sig-nificant, including ones that made little sense. Of course, with an Nof 700 correlations as large as .08 are &dquo;beyond the .05 level.&dquo; Manyof the &dquo;significant&dquo; correlations were of no theoretical or practicalimportance.The point of view taken here is that if the null hypothesis is not

    rejected, it usually is because the N is too small. If enough data isgathered, the hypothesis will generally be rejected. If rejection ofthe null hypothesis were the real intention in psychological experi-ments, there usually would be no need to gather data.The arguments above apply most straightforwardly to &dquo;two-tail

    tests,&dquo; which are used in most experiments. A somewhat better argu-ment can be made for using the null hypothesis in the one-tail test.However, even in that case, if rejection of the null hypothesis isnot obtained for the specified direction, the hypothesis can be re-versed and rejection will usually occur.

    Perhaps my intuitions are wrong-perhaps there are many casesin which different treatments produce the same effects and manycases in which correlations are exactly zero. Even so, the emphasison the null-hypothesis models is unfortunate. As is well recog-nized, the mere rejection of a null hypothesis provides only meagerinformation. For example, to say that a correlation is &dquo;signifi-cantly&dquo; different from zero provides almost no information aboutthe relationship. Some would argue that finding &dquo;significance&dquo; isonly the first step, but how many psychologists ever go beyond thisfirst step?

    Psychologists are usually not interested in finding tiny relation-ships. However, once this is admitted, it forces either a modifica-tion or an abandonment of the null-hypothesis model.

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  • 644 EDUCATIONAL AND PSYCHOLOGICAL MEASUREMENT

    An alternative to the null hypothesis is the &dquo;fixed-increment&dquo;hypothesis. In this model, the experimenter must state in advancehow much of a difference is an important difference. The modelcould be used, for example, to test the differential effect of twomethods of teaching psychology, in which an achievement test isused to measure the amount of learning. Suppose that the regularmethod of instruction obtains a mean achievement test score of 45.In the alternative method of instruction, laboratory sessions areused in addition to lectures. The experimenter states that he willconsider the alternative method of instruction better if, in the longrun, it produces a mean achievement test score which is at least tenpoints greater than the regular method of instruction. Suppose thatthe alternative method actually produces a mean achievement testscore of 65. The probability can then be determined as to whetherthe range of scores from 55 upwards covers the &dquo;true&dquo; value (theparameter) .The difficulty with the &dquo;fixed-increment&dquo; hypothesis-testing model

    is that there are very few experiments in which the increment canbe stated in advance. In the example above, if the desired statisti-cal confidence could not be found for a ten point increment, theexperimenter would probably try a nine point increment, then aneight point increment, and so on. Then the experimenter is nolonger operating with a hypothesis-testing model. He has switchedto a confidence-interval model, which will be discussed later in thearticle.The Small N Fallacy. Closely related to the null hypothesis is

    the notion that only enough subjects need be used in psychologicalexperiments to obtain &dquo;significant&dquo; results. This often encouragesexperimenters to be content with very imprecise estimates of ef-fects. In those situations where the dispersions of responses aresmall, only a small number of subjects is required. However, suchsituations are seldom encountered in psychology. The question,&dquo;When is the N large enough?&dquo; will be discussed later in the article.Even if the object in experimental studies were to test the null

    hypothesis, the statistical test is often compromised by the small X.The tests depend on assumptions like homogeneity of variance, andthe small N study is not sufficient to say how well the assumptionshold. The small N experiment, coupled with the null hypothesis, is

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  • 645JUM NUNNALLY

    usually an illogical effort to leap beyond the confines of limiteddata to document lawful relations in human behavior.The Sampling Fallacy. In psychological experiments we speak of

    the group of subjects as a &dquo;sample&dquo; and use statistical samplingtheory to assess the results. Of course, we are seldom interestedonly in the particular group of subjects, and it is reasonable toquestion the generality of the results in wider collections of peo-ple. However, we should not take the sampling notion too seriously,because in many studies no sampling is done. In many studies weare content to use any humans available. College freshmen arepreferred, but in a pinch we will use our wives, secretaries, janitors,and anyone else who will participate. We should then be a bit cau-tious in applying a statistical sampling theory, which holds onlywhen individuals are randomly or systematically drawn from a de-fined population.The Crucial Experiment. Related to the misconceptions above

    are some misconceptions about crucial experiments. Before thepoints are argued, a distinction should be made between crucial de-signs and crucial sets of data. A crucial design is an agreed-on ex-perimental procedure for testing a theoretical statement. Even ifthe design is accepted as crucial, a particular set of data obtainedwith the design may not be accepted as crucial.

    Although crucial designs have played important parts in someareas of science, few of them are, as yet, available in psychology.In psychology it is more often the case that experimenters pro-pose different designs for testing the same theoretical statement.Experimental designs that apparently differ in small ways oftenproduce different relationships. However, this is not a seriousbother. Antithetical results should lead to more comprehensivetheory.A more serious concern is whether particular sets of experimental

    data can be regarded as crucial. Even when different psychologistsemploy the same design they often obtain different relationships.Such inconsistencies are often explained by &dquo;sampling error,&dquo; butthis is not a complete explanation. Even when the N 8 are large,it is sometimes reported that Jones finds a positive correlation,Smith a negative correlation, and Brown a nil correlation. Theresults of psychological studies are sometimes particular to the ex-

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  • 646 EDUCATIONAL AND PSYCHOLOGICAL MEASUREMENT

    perimenter and the time and place of the experiment. This is whymost psychologists would place more faith in the results of twostudies, each with 50 subjects, performed by different investigatorsin different places, than in the results obtained by one investigatorfor 100 subjects. Then we must be concerned not only with thesampling of people but with the sampling of experimental environ-ments as well. The need to &dquo;sample&dquo; experimental environments ismuch greater in some types of studies than in others. For example,the need probably would be greater in group dynamic studies thanin studies of depth perception.

    What Should Be Done

    Estimation. Hypotheses are really tested by a process of estima-tion rather than with statistical hypothesis-testing models. Thatis, the experimenter wants to determine what the mean differencesare, how large the correlation is, what form the curve takes, andwhat kinds of factors occur in test scores. If, in the long run, sub-stantial differences are found between effects or if substantial cor-relations are found, the experimenter can then speak of the theoreti-cal and practical implications.To illustrate our dependence on estimation, analysis of variance

    should be considered primarily an estimation device. The variancesand ratios of variances obtained from the analysis are unbiased es-timates of different effects and their interactions. The proper ques-tions to ask are, &dquo;How large are the separate variances? and&dquo;How mu...

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