I've been following the example in the R help page: http://stat.ethz.ch/R-manual/R-devel/library/survival/html/frailty.html library(survival); coxph(Surv(time, status) ~ age + frailty(inst, df=4), lung) Here, in this particular example they fixed the degrees of freedom for the random institution effects to be 4. But, how did they decide? This clearly can't be the number of institutions as there are 19 institutions.> length(unique(lung$inst))[1] 19 But, how to decide what degrees of freedom to specify for your random institution effect? What was the reason they chose df=4? Can anyone please direct me (Reference or explanations) how to decide what df to use for the frailty terms? Thank you very much. Dave -- View this message in context: http://r.789695.n4.nabble.com/Method-df-for-coxph-in-survival-package-tp4566006p4566006.html Sent from the R help mailing list archive at Nabble.com.