The standard treatment is the same in both comparison. How do you do a three-level treatment factor? I thought you had to have a censoring indicator which took values 0 or 1 not 1, 2 or 3?
Thanks, Laura On Tue, Aug 26, 2008 at 11:05 AM, Peter Dalgaard <[EMAIL PROTECTED]>wrote: > Laura Bonnett wrote: > > Dear R help forum, > > > > I am using the function 'coxph' to obtain hazard ratios for the > comparison > > of a standard treatment to new treatments. This is easily obtained by > > fitting the relevant model and then calling exp(coef(fit1)) say. > > > > I now want to obtain the hazard ratio for the comparison of two > non-standard > > treatments. > > >From a statistical point of view, this can be achieved by dividing the > > exponentiated coefficients of 2 comparisions. E.g. to compared new > treatment > > 1 (nt1) to new treatment 2 (nt2) we can fit 2 models: > > fit1 = standard treatment vs nt1 > > fit2 = standard treatment vs nt2. > > The required hazard ratio is therefore exp(coef(fit1))/exp(coef(fit2)) > > > > In order to obtain an associated confidence interval for this I require > the > > covariance of this comparison. I know that R gives the > variance-covariance > > matrix by the command 'fit$var'. However, this only gives the covariance > > matrix for non standard drugs and not the full covariance matrix. > > > > Can anyone tell me how to obtain the full covariance matrix? > > > > > What kind of data do you have? Is the "standard treatment group" the > same in both comparisons? If so, why not just have a three-level > treatment factor and compare nt1 to nt2 directly. If the control groups > are completely separate, then the covariance between fits made on > independent data is of course zero. > > > Thank you, > > > > Laura > > > > [[alternative HTML version deleted]] > > > > ______________________________________________ > > R-help@r-project.org mailing list > > https://stat.ethz.ch/mailman/listinfo/r-help > > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > > and provide commented, minimal, self-contained, reproducible code. > > > > > -- > O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B > c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K > (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 > ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 > > > [[alternative HTML version deleted]]
______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
