Testing discrepancy effects: a critique, a suggestion, and an illustration.

To assess the effects of discrepancy between two independent variables, investigators sometimes compute difference scores and correlate such scores with a criterion variable. However, the correlation of the difference with the criterion is accounted for by the correlations of the difference constituents with the criterion and the constituents' variances. It follows that when investigators are testing a prediction that is not captured by the difference constituents' main effects, using the difference correlation analysis may be misleading. Under these circumstances, the effects of a discrepancy between two independent variables can be assessed by a test of their interaction. The problems inherent in using difference scores and the advantage of testing the interaction are illustrated in relation to research programs on two separate topics in social psychology.