I am interested in evaluating whether or not the joint effect of two
independent variables on an outcome are additive. I want to be able to give
something analogous to the Breslow-Day test of homogeneity, which evaluates
effect modification on the multiplicative scale, but on the additive scale.
I would also like to calculate the common Risk Difference across two strata.
Does anyone know of any R package that has these functions, which focus on
additive relationships in stratified analyses?

Cheers,

Jens

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