Sample size calculations for main effects and interactions in case-control studies using Stata's nchi2 and npnchi2 functions

The non-central Chi-squared distribution can be used to calculate power for tests detecting departure from a null hypothesis. Required sample size can also be calculated because it is proportional to the non-centrality parameter for the distribution. We demonstrate how these calculations can be carried out in Stata using the example of calculating power and sample size for case-control studies of gene-gene and gene-environment interactions. Do-files are available for these calculations. Copyright 2003 by Stata Corporation.