Using relative risk models for estimating synergy between two risk factors.

The synergy between two risk factors measures the extent to which each of them is more harmful in combination with the other than when acting alone. Relative risk models which describe interaction between risk factors with a few parameters are well adapted to the quantification of synergy. A simple measure of synergy and a method for its estimation with the help of relative risk models are proposed. This concept of synergy provides a simple interpretation of the non-linear parameter of several classical relative risk models. The discussion is illustrated with examples taken from cancer epidemiology of the upper aerodigestive tract.