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date: 17 February 2019

Abstract and Keywords

An important aspect of study design is sample size planning. However, research questions in psychology and related disciplines can generally be framed from at least two different perspectives: (1) one in which the existence of an effect addresses the question of interest and (2) one in which the magnitude of an effect addresses the question of interest. Correspondingly, depending on which of the two perspectives to addressing a research question is appropriate, there is a different perspective that should be taken to planning an appropriate sample size. In particular, statistical power analysis addresses questions related to the existence of an effect, such as “What size sample is necessary to correctly reject a false null hypothesis with some desired probability?” whereas the accuracy in parameter estimation perspective address questions related to the magnitude of an effect, such as “What size sample is necessary to have a sufficiently narrow confidence interval for the population parameter?” Neither one of these questions is necessarily better than the other, but each addresses a fundamentally different question. This chapter focuses on the interplay of the effect size and the research question of interest to plan an appropriate sample size from either the power analytic or the accuracy in parameter estimation perspective.

Keywords: Effect size, sample size planning, confidence intervals, null hypothesis significance test, power analysis, accuracy in parameter estimation, study design, research design

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