The output is as follows. My question is how to include only the interaction terms where x is equal to "Maintained". (x has two possible values "Maintained" and "Nonmaintained".) -- View this message in context: http://r.789695.n4.nabble.com/coxph-how-to-define-interaction-terms-tp4679162p4679176.html Sent from the R help mailing list archive at Nabble.com.