The fact that difference scores tend to be less reliable
than the original measurements from which they
are calculated should not be a matter of concern in
testing the significance of treatment-induced change.
The reliabilities of the original measurements are important
because unreliability attenuates correlation, and
substantial correlation between prescores and postscores
is required for difference scores to be of value
in controlling for individual differences. Reliability
notwithstanding, difference scores provide superior
control over true baseline differences in quasi-experimental
research, whereas the analysis of covariance
(ANCOVA) is generally preferable for baseline control
in randomized experimental designs. Index terms:
analysis of covariance, baseline correction, difference
scores, measurement of change, reliability.