Risk ratio estimation in case-cohort studies.

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Abstract
In traditional (cumulative-incidence) case-control studies, the exposure odds ratio can be used as an estimator of the risk ratio only when the disease under study is rare. The case-cohort study is a recently developed useful modification of the case-control study. This design allows direct estimation of the risk ratio from a fixed cohort, but does not require any rare-disease assumption. This article reviews recent developments in risk ratio estimation procedures for the analysis of case-cohort data. In the crude analysis, it is shown that the empirical risk ratio estimator is not fully efficient, and the maximum likelihood estimation of the crude risk ratio is discussed. In the stratified analysis, several common risk ratio estimation procedures and standardization methods have been proposed for large strata. However, the Mantel-Haenszel risk ratio and its variance estimator are the only available methods for sparse data.