ExpeRts,

I'm trying to compare three survival curves using the function "survdiff" in the survival package.  Following is my code and corresponding error message.
> survdiff(Surv(st_months, status) ~ strata(BOR), data=mydata)
Error in survdiff(Surv(st_months, status) ~ strata(BOR), data = mydata) :
  No groups to test

When I check the "strata" of the variable.  I get . . .
> strata(BOR)
Levels: BOR=PD BOR=PR BOR=SD

So I know there are levels associated with the BOR variable, but I can't seem to get the strata statement to work within "survdiff".  I checked ?strata and (unless I am not understanding) the help, it seems to be working as I get the following output
> levels(strata(BOR))
[1] "BOR=PD" "BOR=PR" "BOR=SD"

I can get the survival tables and KM plot with no problem, but I can't seem to be able to compare the survival curves.  I can get an overall comparison if I remove the "strata" statement
> survdiff(Surv(st_months, status) ~ BOR, data=mydata)
Call:
survdiff(formula = Surv(st_months, status) ~ BOR, data = mydata)

n=35, 5 observations deleted due to missingness.

        N Observed Expected (O-E)^2/E (O-E)^2/V
BOR=PD 13       13     6.51     6.464      8.87
BOR=PR 10       10    18.48     3.888     10.59
BOR=SD 12       12    10.01     0.395      0.61

 Chisq= 13.5  on 2 degrees of freedom, p= 0.00117

But when I use survdiff(Surv(st_months, status) ~ strata(BOR), data=mydata)  it doesn't want to work.  Any suggestions?

Many thanks for your assistance!


_____________________________________________
Patrick Richardson
Biostatistician - Program of Translational Medicine
Van Andel Research Institute - Webb Lab
333 Bostwick Avenue NE
Grand Rapids, MI  49503

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