Practice of Epidemiology Estimation of the Relative Excess Risk Due to Interaction and Associated Confidence Bounds

The relative excess risk due to interaction (RERI) provides a useful metric of departure from additivity of effects on a relative risk scale. In this paper, the authors show that RERI is identical to the product term in a linear odds ratio or a linear relative risk model. SAS and STATA codes are provided for fitting a linear odds ratio model that directly parameterizes RERI. In addition, this paper presents a method for obtaining likelihood-based 95 % confi-dence bound estimates for RERI. The authors show that likelihood-based confidence intervals may differ sub-stantially from the asymptotic confidence interval estimates advocated by previous authors. The approach presented in this paper should facilitate estimation of RERI and associated likelihood-based confidence bounds, by using standard statistical packages. confidence intervals; interaction; logistic regression; risk ratio Abbreviations: ERR, excess relative risk; RERI, relative excess risk due to interaction; RR, relative risk. Often, in epidemiologic research, a data analyst is inter-ested in the joint exposure effects of 2 factors on disease risk. Questions regarding the nature of joint effects arise in nearly all areas of epidemiologic study, as investigators consider potential interactions between factors that may be biologic