It is widely believed that measures of gain, growth, or change, expressed as simple differences between pretest and posttest scores, are inherently unreliable. It is also believed that gain scores lack predictive validity with respect to other criteria. However, these conclusions are based on misleading assumptions about the values of parameters in familiar equations in classical test theory. The present paper examines modified equations for the validity and reliability of difference scores that describe applied testing situations more realistically and reveal that simple gain scores can be more useful in research than commonly believed.Over a quarter century ago, Cronbach & Furby (1970) and many other authors (e.g., Gulliksen, 1950;Lord & Novick, 1968) concluded that simple differences between pretest and posttest scores have questionable value in behavioral and social science research. Yet this conclusion seems incompatible with the intuition of researchers in many disciplines who assume that measures of gains, changes, differences, growth, and the like are meaningful in experimentation, program evaluation, educational accountability studies, and the investigation of developmental growth and change.During the past two decades, many researchers using difference (or gain) scores have had difficulty justifying the use of such measures, even when they appear to yield interesting and reproducible findings. However, recent research on this topic has provided results favorable to simple gain scores (e.
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