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Sample-Size Requirements for Comparisons of Two Groups on Repeated Observations of a Binary Outcome

Weill Medical College of Cornell University

When preparing a research protocol, an investigator must be as careful in projecting sample-size requirements as in specifying hypotheses. In this article, tables are presented that provide estimates of sample-size requirements for statistical power of 0.80 with two-tailed -levels of 0.05 in studies with a balanced design that plan to compare two groups on time-averaged, repeated observations of a binary outcome. The estimates, which are based on the algorithm of Diggle, Heagerty, Liang, and Zeger, are a function of several features of the study, including the response rates for each group, the number of repeated observations per participant, and the strength of the association among observations within participant as quantified with an intraclass correlation coefficient.

Key Words: power analysis • sample size • repeated measures • binary • mixed-effects logistic regression

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