Assessing the gain in efficiency due to matching in a community intervention study.

COMMIT (Community Intervention Trial for Smoking Cessation) is a randomized study employing a matched pairs design. Pairs of communities were selected on the basis of their geographical proximity and were chosen to be matched on variables strongly expected to relate to the outcome variable, the smoking quit rate. However, quantitative information was not available to evaluate the efficiency gain from matching. We have used baseline smoking quit rates in the communities as a surrogate for the outcome measure to evaluate the gain in efficiency from the matching. Our method takes account of the possible imperfection of the surrogate as a representative of the true outcome. The method estimates an efficiency gain of at least 50 per cent using the matched design. We also evaluate the further gains in efficiency which would be made by using the baseline quit rate to balance the randomization.