Hi David, Thanks for your response. I have discovered that my first and second models differed in length, and have since fixed them. Kirsten -- View this message in context: http://r.789695.n4.nabble.com/Help-with-improveProb-function-in-Hmisc-in-R-tp4713004p4713053.html Sent from the R help mailing list archive at Nabble.com.