Some Practical Guidelines for Effective Sample-Size ...

Some Practical Guidelines for Effective Sample-Size DeterminationRussell V. Lenth Department of StatisticsUniversity of IowaMarch 1, 2001 AbstractSample- size determination is often an important step in planning a statistical study and it is usuallya difficult one. Among the important hurdles to be surpassed, one must obtain an estimate of one ormore error variances, and specify an effect size of importance. There is the temptation to take someshortcuts. This paper offers some suggestions for successful and meaningful Sample-Size discussed is the possibility that sample size may not be the main issue, that the real goal is todesign a high-quality study. Finally, criticism is made of some ill-advised shortcuts relating to powerand sample words:Power; sample size ; Observed power; Retrospective power; Study design; Cohen s effectmeasures; Equivalence testing; I wish to thank John Castelloe, Kate Cowles, Steve Simon, two referees, an editor, and an associate editor for their helpfulcomments on earlier drafts of this paper.

in biostatistics journals, concerning sample-size determination for specific tests. Also of interest are studies of the extent to which sample size is adequate or inadequate in published studies; see Freiman et al. (1986) and Thornley and Adams (1998). There is a growing amount of software for sample-size determination,

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