From: Göran Broström <gb_at_stat.umu.se>

Date: Sun 26 Jun 2005 - 23:52:21 EST

Date: Sun 26 Jun 2005 - 23:52:21 EST

On Fri, Jun 24, 2005 at 11:27:28AM -0400, sms13+@pitt.edu wrote:

*> Hi,
*

> I was wondering if someone can help me

*> interpret the results of running
**> weibreg.
**>
**> I run the following and get the
**> following R output.
**> > weibreg(Surv(time, censor)~covar)
**> fit$fail = 0
**> Call:
**> weibreg(formula = Surv(time,
**> censor)~covar)
**>
**> Covariate Mean Coef
**> Rel.Risk L-R p Wald p
**> covar 319.880 -0.002 0.998
**> 0.000
**>
**> log(scale) 0.000 8.239
**> 3786.326 0.000
**> log(shape) 0.000 0.265
**> 1.304 0.000
**>
**> Events 172
**> Total time at risk 845891
**> Max. log. likelihood -1609.4
**> LR test statistic 34.4
**> Degrees of freedom 3
**> Overall p-value 1.65026e-07
**>
**>
**> I would just like to find the estimated
**> mean survival time as a function of the
**> covariate in the model, but am not sure
**> how to use this output to find that.
**> Any help would be greatly appreciated.
*

The fitted model is a distribution with hazard function

h(t; a, b, z) = (a/b)(t/b)^(a-1)exp(beta*z),

where a = "baseline shape" and b = "baseline scale". z is your "covar" and beta is the estimated regression coefficient. It is an easy exercise to show that this is the hazard function of a Weibull distribution with shape a and scale b*exp(-beta*z/a). Thus the mean is

E(T) = b*exp(-beta*z/a)*gamma(1+1/a) ## See ?Weibull

In your case, a = 1.304, b = 3786.326, beta = -0.002, so

E(T) = 3495 * exp(0.00153 * z)

(given my calculations are correct).

Hth,

Göran

*>
*

> Thank you,

*> Steven
**>
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**>
*

-- Göran Broström tel: +46 90 786 5223 Professor and Head Department of Statistics fax: +46 90 786 6614 Umeå University http://www.stat.umu.se/~goran.brostrom/ SE-90187 Umeå, Sweden e-mail: gb@stat.umu.se ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.htmlReceived on Sun Jun 26 23:58:20 2005

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