Optimal Allocation of Assessment Time in Randomized Pretest–Posttest Designs

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The widely used pretest–posttest design traditionally consists of a pretest and a posttest of equal lengths. However, greater statistical power can usually be achieved by using a posttest that is somewhat longer than the pretest. This article provides a method for calculating the optimal allocation of pretest and posttest assessment units so as to maximize power and presents the accompanying reduction in required sample size for a range of parameter values. When insufficient information is available to estimate the optimal allocation precisely, the effects of allocating 75% of the assessment units to the posttest are evaluated.

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