A form of matching often used when control subjects are more readily obtained than cases. A number, m (m > 1), of controls are attached to each case, these being known as the matched set. The theoretical efficiency of such matching in estimating, for example, relative risk, is m / (m + 1) so one control per case is 50% efficient, while four per case is 80% efficient. Increasing the number of controls beyond 5 10 brings rapidly diminishing returns.