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...ers, I know, this is the second time i return on this topic. Sorry, but this analysis is of great value for me, and i hope someone can help me. I need to model a time-varying effect in a Cox model. Briefly explained here: http://books.google.com/books?id=9kY4XRuUMUsC&lpg=PP1&hl=it&pg=PA147#v=onepage&q&f=false http://finzi.psych.upenn.edu/R/Rhelp02/archive/52828.html It seems that SAS is more well-suited to this purpose, as explained in the links above, but i do not have enough time to learn SAS. I think i should use something like this: cph(Surv(start,end,status)~a.facto...