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Biostatistics 2005 6(1):1-9; doi:10.1093/biostatistics/kxh024
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Biostatistics Vol. 6 No. 1 © Oxford University Press 2005; all rights reserved.

Additive and multiplicative models for the joint effect of two risk factors

A. Berrington de González*

Cancer Research UK Epidemiology Unit, University of Oxford, Gibson Building, Radcliffe Infirmary, Oxford, OX2 6HE, UK amy.berrington{at}cancer.org.uk

D. R. Cox

Nuffield College, University of Oxford, OX1 1NF, UK david.cox{at}nuffield.ox.ac.uk

* To whom correspondence should be addressed.

Simple tests are given for consistency of the data with additive and with multiplicative effects of two risk factors on a binary outcome. A combination of the procedures will show whether data are consistent with neither, one or both of the models of no additive or no multiplicative interaction. Implications for the size of the study needed to detect differences between the models are also addressed. Because of the simple form of the test statistics, combination of evidence from different studies or strata is straightforward. Illustration of how the method could be extended to data from a 2xRxC table is also given.

Keywords: Case-control studies; Cohort studies; Interaction; Multiplicative; Additive


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