Williams & Zimmerman (1996) provided much- needed clarification on the reliability of gain scores. This commentary translates these ideas into recogniz able patterns of change that tend to produce reliable or unreliable gain scores. It also questions the relevance of the traditional idea of reliability to the measurement of change. Index terms: change scores, classical test theory, difference scores, gain scores, intraindividual differences, measurement of growth, reliability, test theory, validity.
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