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Evaluation & the Health Professions
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Obtaining Power or Obtaining Precision

Delineating Methods of Sample-Size Planning

Ken Kelley

University of Notre Dame, KKelley{at}ND.edu

Scott E. Maxwell

University of Notre Dame, SMaxwell{at}ND.edu

Joseph R. Rausch

University of Notre Dame, JRausch{at}ND.edu

Sample-size planning historically has been approached from a power analytic perspective in order to have some reasonable probability of correctly rejecting the null hypothesis. Another approach that is not as well-known is one that emphasizes accuracy in parameter estimation (AIPE). From the AIPE perspective, sample size is chosen such that the expected width of a confidence interval will be sufficiently narrow. The rationales of both approaches are delineated and two procedures are given for estimating the sample size from the AIPE perspective for a two-group mean comparison. One method yields the required sample size, such that the expected width of the computed confidence interval will be the value specified. A modification allows for a defined degree of probabilistic assurance that the width of the computed confidence interval will be no larger than specified. The authors emphasize that the correct conceptualization of sample-size planning depends on the research questions and particular goals of the study.

Key Words: experimental design • sample-size planning • statistical power • statistical precision • accuracy in parameter estimation

Evaluation & the Health Professions, Vol. 26, No. 3, 258-287 (2003)
DOI: 10.1177/0163278703255242
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A. K. Singh, K. Kelley, and R. Agarwal
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