Contradictions Can Never a Paradox Resolve

The fact that difference scores tend to be less reli able 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 im portant because unreliability attenuates correlation, and substantial correlation between prescores and post- scores 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-experi mental 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.