Survival analysis with no events in one treatment group

Hi John,

to me it still seems that you have two "problems", given your first email.
The first one, if I am correct, is that you have NO admissions in one of the
groups. That is, you have Treat A, B, C, D and for one of these treatments
there is not a single admission. Then you cannot estimate a survival
function for this group because nobody died (got hospitalized). I think you
have to exclude this group because your survival rate is 100% and your
hazard rate 0%. But more proficient people may have better advice with that.

That subject 2 did not get hospitalized is not a problem though. The
observation is censored at 12 months. Since there are more patients in his
treatment group (subject 1) who had an event, the survival function can be
estimated. 

To the multiple admissions problem: To account for the fact that some have
more than one event you may create a "number of events" variable, say
numEvents, and include that in a strata() argument to your regression call:

surv.obj<-Surv(time=time1,time2=time2,event=event,type="counting")
model<-coxph(surv.obj~Treatment+strata(numEvents))

Use strata if you think that your baseline hazard is different in the
different strata (in your case: if you think that the baseline hazard of
having an event differs with having had prior event(s)). At the same time
you assume that your treatment effect  - the beta on your Treat variable -
is the same across all strata. If you have reason to assume that the effect
of your treatment varies (interacts) with the number of prior events, then
it is not the correct approach. In addition you may include the
cluster(Subject) or frailty(Subject) commands.

Instead of strata you might also consider using a dummy variable, coding the
number of prior events (if the maximum number admissions per patient is
reasonably small and the number of cell frequencies reasonably large).

Finally, you may want to consult Terry Thernau and Patricia Grambsch's book
"Modeling survival data". It shows how to apply the techniques in R/S-Plus.
I find it invaluable.

Does that help you?

Daniel

-------------------------
cuncta stricte discussurus
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-----Urspr?ngliche Nachricht-----
Von: John Field [mailto:JohnField at ozemail.com.au] 
Gesendet: Sunday, December 30, 2007 10:14 PM
An: Daniel Malter; R-help at r-project.org
Betreff: Re: AW: [R] Survival analysis with no events in one treatment group

Hi Daniel,

Sorry, it may have been clearer if I had used "subjects" instead of
"patients".  The treatments were administered to all subjects, and then in
the succeeding 12 months, some were hospitalised and some were not.  Hence
only about 25% of the subjects were hospitalised.

The start of the data:
subj    time1   time2   event Treat
1       0       6.2     1       A
1       6.2     12      0       A
2       0       12      0       A

so subject 1 was hospitalised at 6.2 months, subject 2 not at all.

Hope this makes it clearer.
Regards,
John

Survival analysis with no events in one treatment group

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