On Fri, 31 Mar 2006, singyee ling wrote:
> Dear all,
>
> I use the Andersen plot to check for proportional hazards assumption for a
> factor (say x) in the Cox regression model and obtained a straight line that
> pass through the origin. However, the formal test done by the R-function
> cox.zph, which is based on the plot of Schonefeld residuals against time,
> indicates that proportional hazards assumption is violated. Further, a plot
> of the score process (cumulative sums of schoenfeld residuals) against time
> again give the same conclusion as the cox.zph function and i am really
> stumped by this. Klein et al (1997, pp 354) mentioned that graphical checks
> for proportional hazards assumption is often preferred to formal test as
> formal test based on a large enough sample (in my case is 4000 data
> entries), will often reject the null hypothesis of proportionality. Is that
> what is happening in my case? any suggestion?
>
If you plot the cox.zph object you should be able to see why the test is
rejecting. The plot includes a smooth curve that estimates the log hazard
ratio as a function of time, and you can see if you think it departs
importantly from a horizontal line.
The cox.zph tests are (approximately) score tests against particular
one-dimensional alternatives and so are more powerful against those
alternatives than a test based on the supremum of the score process.
-thomas
Thomas Lumley Assoc. Professor, Biostatistics
[EMAIL PROTECTED] University of Washington, Seattle
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