Factors influencing the optimal control-to-case ratio in matched case- control studies

Statistical power in matched case-control studies depends on both the correlation coefficient between cases and their matched controls (phi) and the prevalence of exposure among controls (P₀). To examine the hypothesis that the value of increasing the control-to-case ratio beyond 5 varies with both phi and P₀, the authors estimated statistical power for a hypothetical case-control study under different assumptions. The effect of increasing the control-to-case ratio depended on phi and, to a lesser extent, on P₀. The results suggest that investigators consider including more than five controls per case when either phi is greater than about 0.2 or P₀ is less than about 0.15.