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.