Dear List, Is the test for proportional hazards valid when the model contains a cluster variable? The output looks strange with the cluster variable. My intervals are based on calendar time and the clustering variable is related to the season the event occurs in. model1<-coxph(Surv(Start,Stop,Event)~LagAOO+I(LagAOO^2)+ FirstSeen+TSLE+strata(CumPOO.rc)+cluster(quarter),data=data8, x=T) >Cox.zph(model1) LagAOO 0.209 5.61 0.0179 I(LagAOO^2) -0.209 5.61 0.0179 FirstSeen 0.209 5.61 0.0179 TSLE 0.209 5.61 0.0179 GLOBAL NA 5.61 0.2304 In this example there is strong evidence for non-proportional hazards for each of my covariates. By defining my strata differently I can remove the non-proportional hazards. A quick comment is appreciated. Alex [[alternative HTML version deleted]]