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I want to use gamm to generate smoothed trend line for three groups identified by dummy variable genea and geneb. My question is how to add in an interaction term between the time and another dummy variable such as gender? fitm<-gamm(change_gfr~ genea+geneb+s(timea_n,bs="ps")+s(timeb_n,bs="ps")+s(timec_n,bs="ps"),data=mm,random=list(time_n=~1|PID)) -- View this message in context: http://r.789695.n4.nabble.com/how-to-add-in-interaction-terms-in-gamm-tp3400243p3400243.html Sent from the R help mailing list a...