Matched Case-Control Studies with a Variable Number of Controls per Case

SUMMARY Previous authors have analysed case-control studies where the cases are individually matched to a fixed number of controls. This is extended here to allow the matching ratio to vary; this can occur by design, by censoring, or in an interim data analysis. Statistics are derived to test the case-control difference for both binary and continuous variables; the power functions and required sample sizes are also given. Finally the effect of the matching ratio on cost-efficiency is discussed. Examples of the calculations are shown using data on Hodgkin's disease.