Degree of matching and gain in power and efficiency in case-control studies.

Frequency matching can be used to increase the precision and power of case-control studies. Unmatched and frequency-matched designs are only two distinct possibilities of control selection in a continuum regarding the frequency of the matching factor in controls. We assessed the power and efficiency of case-control studies under a variety of assumptions regarding the prevalence and the effects of the matching factor and the exposure of interest as well as their association in the population. For each set of parameters, we simulated 10,000 case-control studies varying the degree of matching, that is, the proportion of the matching factor in selected controls over a wide range including the proportion in cases (matched design) and the population (unmatched design) as two special options. Traditional frequency matching increased the precision and power in most scenarios, but most of the gain was often achieved by incomplete (less than perfect) matching. Even greater gains were sometimes observed by increasing the prevalence of the matching factor in controls above the one in cases. In the scenarios assessed, perfect matching was neither necessary nor the optimum degree of matching in many circumstances. It might be worthwhile to evaluate the optimum degree of matching for specific settings in the design of case-control studies.