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From |
rgutierrez@stata.com (Roberto G. Gutierrez, StataCorp) |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Just what are -tvc- and -texp- doing in -stcox-? |

Date |
Fri, 31 Jul 2009 10:34:01 -0500 |

Jocelyn Andrel <Jocelyn.Andrel@mail.jci.tju.edu> asks: > I'm having trouble with -stcox-. I need to produce predicted survival > curves, but my model has a time-dependent covariate, so I can't use -tvc- > and -texp-, I have to use -stspilt-, then -stcox-. Which would be fine, > except my unsplit and split model simply don't match. Here's the background: > the data is time to death from a cancer, the time-dependent covariate is > radiation after surgery. Some people had it, some didn't, so the radiation > variable is dichotomous (0/1) for radiation, then the time of radiation is > there for those who had it, and missing for those who didn't. > I really have two questions: > 1. How do -tvc- and -texp- work? > 2. What's wrong with this code? > My code looks like: > stset timetoevent, failures(censor) id(id) > stcox age, tvc(rad) texp(timeofrad) > or > stsplit at(failures) > gen radtvc = rad*(_t > timeofrad) > stcox age radtvc -tvc()- and -texp()- work by interacting the variables in -tvc()- with the time expression specified in -texp()-. For example . stcox age, tvc(rad) texp(_t > 10) would be equivalent to . stsplit, at(failures) . gen radtvc = rad * (_t > 10) . stcox age radtvc However, what Jocelyn is encountering is that the equivalence vanishes when the constant 10, say, is replace by the variable -timeofrad-. The reason is that, given the time-varying nature of -texp()-, the values of -timeofrad- correspond to observations whose analysis times coincide with observed failures times, not with the observations as given in the original data. As such, -texp()- should only be used with functions of analysis time that do not include variables. This is also mentioned in the Technical Note on page 134 of the Stata 11 [ST] manual. That stated, an error message would be more appropriate here. We'll make that change as part of a future ado update. --Bobby rgutierrez@stata.com * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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