[R] p-value=0 running coxph

Thomas Lumley tlumley at u.washington.edu
Thu Oct 30 21:44:31 CET 2008

On Thu, 30 Oct 2008, GSt wrote:

> Dear all,

>I have a question concerning the p-value. When running coxph I get a p-value
>= 0. :confused:
>Can this be true? Why arenÿÿt there decimal points? Is there a way to 
>find out the exact p-value?

The p-values are rounded, and of course they are not exactly zero, just 
<0.0005.  You are given the z-statistic so you could compute the p-value

>  2*pnorm(-9.16)
[1] 5.189732e-20

Now, this number is probably pretty useless because the Normal 
approximation to the test statistic doesn't work that far out in the tails 
unless you have a very large number of events. It's also unusual for the 
null hypothesis to be even slightly interesting when the p-value is that 
small -- do you really not know whether albumin concentrations are related 
to the outcome?


Here is the output:
coxph(formula = Surv(start, stop, status) ~ Albumin_gproL, data = dial,
na.action = na.omit, method = "breslow")

n=22449 (31 observations deleted due to missingness)
                        coef   exp(coef)   se(coef)         z    p
Albumin_gproL -0.157         0.855     0.0172   -9.16    0

                        exp(coef)    exp(-coef)    lower .95    upper .95
Albumin_gproL         0.855             1.17         0.826         0.884

Rsquare= 0.003   (max possible= 0.033 )
Likelihood ratio test = 73.6  on 1 df,   p=0
Wald test               = 83.8  on 1 df,   p=0
Score (logrank) test = 86.1  on 1 df,   p=0

> cox.zph(cox_1111)
                           rho    chisq          p
Albumin_gproL -0.0509    0.197    0.657

Thank you in advance...
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Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle

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