Contradictions can never a paradox resolve

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