I found a nice presentation which I think addresses my question/problem. See slide 3 here: http://www.ispor.org/meetings/atlanta0510/presentations/IP1-CookJohnR.pdf -- View this message in context: http://r.789695.n4.nabble.com/Quantitative-analysis-of-treatment-effects-tp4635094p4635095.html Sent from the R help mailing list archive at Nabble.com.