laura bonnett ljbonnettgooglemailcom sorry last email appeared missing important bits ill try dear currently working coxph function within package survival model h_ij h_0t exp b1x1 b 2x2 indicator variables follows x1 x2 0 0 b 1 0 c 0 1 hazard rates b h_ijth_0texpb1 c h_ijth_0texpb2 h_ijth_0t therefore means hazard ratios ba h_0texpb1h_0 expb1 ca expb2 bc expb1expb2 using coxph easy determine ba ca associated confidence intervals fairly trivial obtain hazard ratio bc isolating exponentiated coefficients previously fitted models dividing however work determine confidence interval bc know clear variancecovariance matrix obtain looked functions vcoc var problem ba called fit1 ca called fit2 appears easy way combine fitted models produce required matrix thank help laura ps sorry 1st attempt alternative html version deleted ______________________________________________ rhelprprojectorg mailing list httpsstatethzchmailmanlistinforhelp please read posting guide httpwwwrprojectorgpostingguidehtml provide commented minimal selfcontained reproducible code wed 06 aug 2008 124706 0100 r variancecovariance matrix parameter estimates