Distinguishing between measurements and dependent variables

Abstract

Humphreys and Drasgow (1989b) recognize two types of dependent variables: the original measurements collected in an experiment and mathematical variables that are subjected to statistical analysis. Overall and Woodward (1975) were explicitly concerned with the latter, whereas Humphreys and Drasgow contend that they were concerned with reliability of the original measurements from which difference scores may be computed. These are quite different matters. Criticisms should focus on points of disagreement, and there has never been any disagreement concerning the importance of reliability of the original measurements. The notion that treatment effects should be considered a part of the true variance for calculation of reliability estimates is rejected as stemming from their failure to understand the basic difference between reliability and validity. Index terms: control of individual differences, difference scores, measurement of change, reliability of the marginal distribution, statistical power, within-group reliabilities.