Methodological issues in cohort studies: point estimators of the rate ratio.

Computer simulation has been used to study the behaviour of four estimators of the common rate ratio estimated from cohort data, in terms of bias and root mean square error. It is concluded that the maximum likelihood estimator is the preferable approach, though an excellent approximation to this is yielded by the analogue of the Mantel-Haenszel estimator. When the number of observed cases in the referent group is large, it may be preferable to present the results as the standardized mortality (morbidity) ratio, though under these circumstances all four estimators tend to the same value. The O/E estimator proposed for use with clinical trials data under some situations is noticeably biased and inefficient and therefore should be avoided.