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Sample Size for Prospective and Retrospective Studies: The 2 × 2 Table

Published online by Cambridge University Press:  21 June 2016

Louise-Anne McNutt  and
Robert F. Woolson
Louise-Anne McNutt
Affiliation:
Division of Biostatistics, College of Medicine, The University of Iowa, Iowa City, Iowa
Robert F. Woolson*
Affiliation:
Division of Biostatistics, College of Medicine, The University of Iowa, Iowa City, Iowa
*
Division of Biostatistics, 2811A Steindler Bldg, University of Iowa Hospitals and Clinics, Iowa City, IA 52242
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Extract

Previous papers in this series have discussed the design of epidemiologic studies and analysis for data that can be presented in a 2 × 2 table. The purpose of this paper is to explain how sample sizes are determined for unmatched prospective and retrospective studies. One of the most common questions asked in planning a project is “What sample size do I need for my study?” Determining the correct sample size is an important consideration that is best resolved prior to data collection. In this paper we assume the goal of the study is to examine the association between a dichotomous risk factor and the presence or absence of disease; therefore, the analysis will be that of a 2 × 2 table. We also assume throughout that sample sizes are to be equal for the two comparison groups (ie, equal numbers of exposed and unexposed for a prospective study, and equal numbers of cases and controls for a retrospective study). Sample size curves used to determine sample sizes will be presented. The results here are based on a paper by Schlesselman that contains sample size formulae for these study designs, and to which the reader may refer for an excellent technical discussion of their derivation.

Type
Special Sections
Information
Infection Control & Hospital Epidemiology , Volume 9 , Issue 12 , December 1988 , pp. 562 - 566
Copyright
Copyright © The Society for Healthcare Epidemiology of America 1988

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References

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