Frequency matching can be used to increase the precision and power of case-
control studies. Unmatched and frequency-matched designs are only two disti
nct possibilities of control selection in a continuum regarding the frequen
cy of the matching factor in controls. We assessed the power and efficiency
of case control studies under a variety of assumptions regarding the preva
lence and the effects of the matching factor and the exposure of interest a
s well as their association in the population. For each set of parameters,
we simulated 10,000 case-control studies varying the degree of matching, th
at is, the proportion of the matching factor in selected controls over a wi
de range including the proportion in cases (matched design) and the populat
ion (unmatched design) as two special options. Traditional frequency matchi
ng increased the precision and power in most scenarios, but most of the gai
n was often achieved by incomplete (less than perfect) matching. Even great
er gains were sometimes observed by increasing the prevalence of the matchi
ng factor in controls above the one in cases. In the scenarios assessed, pe
rfect matching was neither necessary nor the optimum degree of matching in
many circumstances. It might be worthwhile to evaluate the optimum degree o
f matching for specific settings in the design of case-control studies.