Assimilation-Contrast Effects and Item Selection in Thurstone Scaling

Previous research shows that Thurstone, Guttman, and Likert scales constructed from a common pool of items differentially select items having a reference to self, and furthermore that prediction to behavior is related to the proportion of self-reference items in the scale. In the present study, the systematic underselection of self-reference items in Thurstone's successiveinternal measurement, is replicated on a new subj ect population. This underselection is explained by showing that assimilation-contrast effects occur even when the usual criteria for eliminating judges for incompetence are employed, and that such effects are most pronounced for self-reference items. It is also suggested that whether a judge's own attitude is a popular or an unpopular one in the social environment is related to assimilation-contrast effects and thus to the type of item appearing in a Thurstone instrument. For years much attitude research has relied upon Thurstone scaling procedures to construct measuring instruments. Questions have been raised, however, about the adequacy of some of these procedures. One recent investigation compared the predictive power of a Thurstone successive-interval scale, a Likert scale, and a Guttman scale, all of which were developed from a common pool of items. The three instruments were designed to measure the same attitude and were evaluated in terms of their utility for predicting behavior patterns. The Thurstone scale proved to be the poorest; the Guttman scale performed somewhat better; and the Likert scale proved to be the best predictive tool. It was suggested that the poor performance of the Thurstone procedure could be accounted for by certain intrinsic features of the Thurstone technique. It was reported that relative to the other techniques the Thurstone method resulted in underselection of items which contain a specific reference to self. It was found that the proportion of selfreferent items included in each type of scale was closely related to the relative predictive power of the three scales.' The present research was undertaken to determine whether the underselection of self-referent items by the Thurstone procedure is replicable using a different subject population. Since the replication was successful, the data were further analyzed in an attempt to specify why the Thurstone procedure results in this systematic underselection of self-specific items. A satisfactory answer to this question would provide a basis for improving attitude measurement, as well as suggest some ideas for the development of a theory of measurement. Our working hypothesis, in explaining the underselection of self-specific items by the Thurstone technique stems from the research of Sherif and Hovland.2 They found evidence of an assimilation-contrast effect when subjects are asked to perform a judging task. A judge tends to assimilate, or perceive a stimulus closer to an internal anchor, whenever a discrepancy between the two is small. A contrast effect involves a movement away from an internal position when the discrepancy between that position and the stimulus is great. In the Sherif and Hovland data this process was manifest in a marked tendency for judges with extreme attitudes to place items in extreme categories. Additional studies have substantiated the influence of assimilation-contrast effects in judging 1 Charles R. Tittle and Richard J. Hill, "Attitude Measurement and Prediction of Behavior: An Evaluation of Conditions and Measurement Techniques," Sociometry, 30 (June 1967), pp. 199-213. 2 Muzafer Sherif and Carl I. Hovland, Social Jtudgment (New Haven: Yale University Press,