Difference Scores From the Point of View of Reliability and Repeated-Measures ANOVA: In Defense of Difference Scores for Data Analysis

There is such doubt in research practice about the reliability of difference scores that granting agencies, journal editors, reviewers, and committees of graduate students' theses have been known to deplore their use. This most maligned index can be used in studies of change, growth, or perhaps discrepancy between two measures taken on the same sampling unit. The most commonly stated problem with difference scores is the supposed associated increase in unreliability of difference scores. In this article, the authors examine difference scores from the point of view of reliability and repeated-measures ANOVA. The authors demonstrate that when the focus of difference scores is data analysis of aggregate models, their use should not be assessed in terms of reliability, and that the complete abolition of difference scores in research practice is unwarranted.