Paradoxes, contradictions, and illusions

Abstract

There is no contradiction between a powerful significance test based on a difference score and the necessity for reliable measurement of the dependent measure in a controlled experiment. In fact, the former requires the latter. In this paper we review the conclusions that were drawn by Humphreys and Drasgow (1989) and show that Overall’s (1989) "contradiction" is an illusion derived from imprecise language. Index terms: analysis of covariance, baseline correction, control of individual differences, difference scores, measurement of change, reliability of the marginal distribution, statistical power, within-group reliabilities.