I used following command:> frailtyPenal(Surv(time, status) ~var1 + cluster(family), Frailty=TRUE > ,n.knots=8, kappa1=1500,+ cross.validation=FALSE) And got this error : Error in Surv(time, status) : Time variable is not numeric In addition: Warning message: In is.na(time) : is.na() applied to non-(list or vector) of type 'closure' ----------- It looks like you forgot data= in your model statement, which makes 'time' refer to the function of that name. Terry T.