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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