Considerations in determining matching criteria and stratum sizes for case-control studies.

Despite considerable research into the statistical properties of matching in case-control studies, much disagreement remains regarding practical guidelines for the employment of the procedure. We critically examine some recent statistical recommendations, and review a number of theoretical and practical considerations that should be employed in designing matched studies. We conclude that determination of matching criteria and stratum sizes should always involve practical as well as theoretical considerations, and that global recommendations regarding matching cannot be justified on the basis of statistical considerations alone.