Thanks. I was able to get what I wanted by doing this: predxn <- function(s,d) { coef(m3)[1] + coef(m3)[2]*s + coef(m3)[3]*s^2 + coef(m3)[4]*d + coef(m3)[5]*d^2 } But it's not very elegant... // joseph w. clark , phd candidate \\ usc marshall school of business