On Thu, 22 Oct 2009, Mura Tamakou wrote:
Dear All,
I have a question regarding the output of survfit() when I supply a Cox model.
Lets say for example:
<snipped: code that doesn't quite run>
we observe that the 95% CIs overlap!! How is this possible since the HR for
spiders is significant.
It's perfectly natural.
To a good approximation, the p-value for the comparison will be significant
when the point estimate for each group is outside the confidence interval for
the other group.
Suppose you had two point estimates with standard error equal to 1.0. The
standard error of the difference would be 1.414, so the p-value would be less
than 0.05 if the two point estimates differ by more than 1.96*1.414. The
confidence intervals will overlap if the point estimates differ by less than
1.96*2.
-thomas
Thomas Lumley Assoc. Professor, Biostatistics
tlum...@u.washington.edu University of Washington, Seattle
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