The Common risk difference and effect modification on the additive scale

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?

It would be possible to use glm with a quasi() family argument with  
link="identity" and variance="mu". This would make the effect  
relationships linear in their untransformed scales but allow the  
variance to be Poisson.

The mgcv package is the one suggested by Simon Wood for what might be  
called "basic" generalized additive models described in his book in  
chapter 5, but there are a large number of other packages that handle  
generalized additive models of various sorts.

http://search.r-project.org/cgi-bin/namazu.cgi?query=%22additive+models%22&max=100&result=normal&sort=score&idxname=functions&idxname=Rhelp08&idxname=views

David Winsemius, MD
Heritage Laboratories
West Hartford, CT