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Educational and Psychological Measurement
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The Unseen Power Loss: Stemming the Flow

Raphael Gillett

University of Leicester

The widespread practice of substituting an empirical estimate of effect size in the standard sample size formula causes the average power of studies to fall below the nominal level,oftenby a considerable margin. Ordinarily concealed from view in an individual experiment, the power loss becomes visible when studies are examined collectively. To help researchers avoid the power deficit, the expected-power method of sample size determination is described. When information from an earlier study is available, it is recommended that researchers should base sample size calculations on the expected-power formula instead of the traditional power formula.

Educational and Psychological Measurement, Vol. 62, No. 6, 960-968 (2002)
DOI: 10.1177/0013164402238084
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