### sas >> Cox model assumption check

by **Larry, Raymond** » Tue, 20 Dec 2005 22:49:53 GMT

I need to check the assumption of Cox model for the treatment group,

which is the main variable of interest, and there are also quite a lot

covariates in the data set. The follow-up time (survival time) is days

from randomization to either diagnosis of disease or censoring.

Is there a way that I can perform this assumption check in SAS? I

thought I can do a plot of residual of survivial time against time?

Will this work?

I appreciate any input from you folks!!!

### sas >> Cox model assumption check

by **davidlcassell** » Wed, 21 Dec 2005 07:16:58 GMT

Before I get to PROC PHREG, I usually check the proportional hazards

assumption

using PROC LIFETEST.

If the proportional hazards assumptionl is appropriate, then the estimates

of the

log(-log(estimated survival distribution)) plotted against the log(TIME)

should be parallel lines.

If the Weibull model is a good fit, then these lines should be pretty

straight too.

If you have SAS 9.1 then you can plot this out using ODS Statistical

Graphics.

HTH,

David

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### sas >> Cox model assumption check

by **jweedon** » Wed, 21 Dec 2005 07:56:16 GMT

On 20 Dec 2005 06:49:53 -0800, "Larry, Raymond" < XXXX@XXXXX.COM >

Which assumption are you referring to? If you mean proportional

hazards, the usual methods are to introduce terms involving

interactions of time or log(time) & the covariates. You may want to

categorize the time variable.

If these terms are important you'll want to retain them.

JW

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