Thank you for your reply, Frank. %ia% drops the product x1'*x2', but keeps x1*x2' and x1'*x2. Is there a way to drop all three of these and include only x1*x2? Or is this not a sensible thing to want to do?
Thanks, Mark Frank E Harrell Jr <f.harr...@vanderbilt.edu> wrote: For the first example you want the restricted interaction operator: y ~ rcs(x1, 3) + rcs(x2, 3) + rcs(x1, 3) %ia% rcs(x2, 3). For the second example use pol(x,2) or something like pol(x1,2) + pol(x2,2) + pol(x1, 2) %ia% pol(x2, 2) If you have to create new variables for R formulas you're usually doing something wrong. Frank Mark Seeto wrote: Dear R help, I'm having some trouble with model formulas for the ols function in the rms package. I want to have two variables represented as restricted cubic splines, and also include an interaction as a product of linear terms, but I get an error message. library(rms) d <- data.frame(x1 = rnorm(50), x2 = rnorm(50), y = rnorm(50)) ols(y ~ rcs(x1,3) + rcs(x2,3) + x1*x2, data=d) Error in if (!length(fname) || !any(fname == zname)) { : missing value where TRUE/FALSE needed ols(y ~ rcs(x1,3) + rcs(x2,3) + I(x1*x2), data=d) Error in if (!length(fname) || !any(fname == zname)) { : missing value where TRUE/FALSE needed I get the same error if I try to fit a model with a quadratic term: ols(y ~ x1 + I(x1^2), data=d) Error in if (!length(fname) || !any(fname == zname)) { : missing value where TRUE/FALSE needed ols(y ~ I(x1^2), data=d) # No error message, but lacks linear term Is there a way to do these things without first creating new variables in the data frame? Thanks, Mark Seeto National Acoustic Laboratories ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. « [hide part of quote] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.