HOW TO EVALUATE INTERACTION BETWEEN CAUSES - A REVIEW OF PRACTICES INCARDIOVASCULAR EPIDEMIOLOGY

To increase the knowledge of interaction or synergy between risk facto rs is an important task in medical research. Still, current literature in cardiovascular epidemiology reflects major misconceptions as how t o evaluate interaction. This paper presents Rothman's model of causati on from which strict empirical criteria of interaction can be derived, In principle, the method to apply consists of comparing risk differen ces for one risk factor of interest across strata of the other. Common ly used but incorrect approaches are exemplified and discussed. These include reporting risk of disease among those with combined exposure, comparing relative risks for one exposure after stratification by leve l of the other, and including an interaction term in the regression mo del and drawing conclusions from its P-value.