| Abstract: | Demonstrates that skew will reduce the test-retest reliability of otherwise normal data and that the greater the skew is, the greater will be the decrease in reliability. In the first section, equations are derived for the test-retest reliability of lognormal data, a skewed distribution that can be returned to normality by the log transformation. The second analytic treatment derives equations for reliability with power transformations (xa) that will produce skew in originally normal data. Finally, analysis of test-retest correlations of actual skewed measures confirmed that if skew was minimized by transformation toward symmetry, then the reliabilities increased; the increase in reliability was related to the extent of skew. Researchers are cautioned to consider transforming data that are skewed to obtain greater symmetry and, thus, improve the test-retest reliability of the resulting measure. (PsycINFO Database Record (c) 2010 APA, all rights reserved) |
