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?
Thank you,
Laura
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