Did you create the 'status' variable the way indicated on p. 797?
Frequently with Surv() it pays to use syntax such as Surv(death,
status==1) to make a clear logical statement of what is an event
(status==1) vs. censored.
PS. Next time include head(seedlings) and str(seedlings) to make
clear what you are using as data.
At 06:51 AM 6/28/2011, Jacob Brogren wrote:
Hi,
I ran the example on pp. 799-800 from Machael Crawley's "The R Book"
using package survival v. 2.36-5, R 2.13.0 and RStudio 0.94.83. The
model is a Cox's Proportional Hazards model. The result was quite
different compared to the R Book. I have compared my code to the
code in the book but can not find any differences in the function
call. My results are attached as well as a link to the results
presented in the book (link to Google Books).
When running the examples on pp. 797-799 I can't detect any
differences in results so I don't think there are errors in the data
set or in the creation of the status variable.
---------------------------------
Original from the R Book:
http://books.google.com/books?id=8D4HVx0apZQC&lpg=PA799&ots=rQgd_8ofeS&dq=r%20coxph%20crawley&pg=PA799#v=onepage&q&f=false
---------------------------------
My result:
> summary(model1)
Call:
coxph(formula = Surv(death, status) ~ strata(cohort) * gapsize,
data = seedlings)
n= 60, number of events= 60
coef
exp(coef) se(coef) z Pr(>|z|)
gapsize -0.001893 0.998109 0.593372
-0.003 0.997
gapsize:strata(cohort)cohort=September 0.717407 2.049112 0.860807
0.833 0.405
exp(coef) exp(-coef) lower
.95 upper .95
gapsize 0.9981 1.002
0.3120 3.193
gapsize:strata(cohort)cohort=September 2.0491 0.488
0.3792 11.074
Rsquare= 0.022 (max possible= 0.993 )
Likelihood ratio test= 1.35 on 2 df, p=0.5097
Wald test = 1.32 on 2 df, p=0.5178
Score (logrank) test = 1.33 on 2 df, p=0.514
Anyone have an idea why this is occurring?
Kind Regards
Jacob
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