This paper proposes sequential estimators for the relative risk, odds ratio, log relative risk or log odds ratio of a dichotomous attribute in two populations. The estimators take the same number of observations from each population, and ensure that the relative mean-square error for the relative risk or odds ratio, or the mean-square error for their logarithmic versions, is less than a given target, regardless of the value of the estimated parameter. The estimation efficiency, computed with respect to the Cramér-Rao bound, is high when the considered attribute is rare or moderately rare.

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