title: Obtaining power or obtaining precision. Delineating methods of sample-size planning.
abstract: 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.
authors.affiliation: Department of Psychology, University of Notre Dame, Notre Dame, IN 46556, USA. KKelley@ND.edu
authors.forename: Ken
authors.initials: K
authors.lastname: Kelley
authors.forename: Scott E
authors.initials: SE
authors.lastname: Maxwell
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Rausch
mesh_terms: D000704:Analysis of Variance; D016001:Confidence Intervals; D003627:Data Interpretation, Statistical; D006302:Health Services Research / Q000379:methods / Q000706:statistics & numerical data; D006801:Humans; D015233:Models, Statistical; D012107:Research Design; D018401:Sample Size; D014481:United States