additive interaction definition
GEIRA calculates measures of both additive and multiplicative interaction. Multiplicative interaction refers to an interaction term in the logistic regression model. Additive interaction is defined as a deviation from additivity of the absolute effects of two risk factors as originally described by Rothman [43,44]. Three measures of interaction on an additive scale, that is, RERI, the relative excess risk due to interaction; AP, the attributable proportion due to interaction; and S, the synergy index are calculated.