[R] Obtaining value of median survival for survfit function to use in calculation

[Polwart Calum (County Durham and Darlington NHS Foundation Trust)]

Hi,

I'm sure this should be simple but I can't figure it out!  I want to get the median survival calculated by the survfit function and use the value rather than just be able to print it.  Something like this:

library(survival)
data(lung)
lung.byPS = survfit(Surv (time, status) ~ ph.ecog, data=lung)
# lung.byPS
  Call: survfit(formula = Surv(time, status) ~ ph.ecog, data = lung)
  1 observation deleted due to missingness
              n events median 0.95LCL 0.95UCL
  ph.ecog=0  63     37    394     348     574
  ph.ecog=1 113     82    306     268     429
  ph.ecog=2  50     44    199     156     288
  ph.ecog=3   1      1    118     Inf     Inf

What I want is to be able to call the median using something like:

lung.byPS$median[ph.ecog=0]

so that I can add it to a plot like this:
                plot (lung.byPS,
                                conf.int=F,
                                lty=1:4,
                )
                abline(h=0.5, lty=5)
                abline(v=lung.byPS$median[ph.ecog=1])
                abline(v=lung.byPS$median[ph.ecog=2])

Anyone got any easy solutions?  Its fairly normal to plot across and down to show medians on survival curves... so I'm sure it must be possible to automate.

Ta

Calum



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