Abstract
This article is concerned with computing the total sample size required for a two-sample comparison when the sizes of the two groups to be compared cannot be fixed in advance. This is frequently encountered when group membership depends on a variable which is observable only after the subject is enrolled to the study, such as a genetic or a biological marker. The most common way of circumventing this problem is assuming a fixed number for the prevalence of the condition that will determine the group membership and compute the required sample size conditionally. In this article this practice is formalized by placing a prior distribution on the prevalence which results in an analytically tractable formula for the unconditional sample size. In particular a sample size inflation factor, a number that can be multiplied with conditional sample size, is presented. An example is given from the planning of a clinical trial investigating the prognostic role of molecular markers in gastrointestinal stromal cancer.
Disciplines
Clinical Trials
Suggested Citation
Gonen, Mithat, "Computing the Total Sample Size When Group Sizes Are Not Fixed" (August 2005). Memorial Sloan-Kettering Cancer Center, Dept. of Epidemiology & Biostatistics Working Paper Series. Working Paper 4.
https://biostats.bepress.com/mskccbiostat/paper4