Dear all -
Following up my own post, having found a Terry Therneau post about the
value of predict(model,type="quantile"),
http://tolstoy.newcastle.edu.au/R/e4/help/08/03/5335.html
the following code ammendment produces what I was intending to see.
It clear that the 0.5 quantile is the inflection point. And predict()
has the value of being distribution independent, and producing valid
SE estimates.
data(lung)
lung.wbs <- survreg( Surv(time, status)~ 1, data=lung, dist='weibull')
curve(pweibull(x, scale=exp(coef(lung.wbs)), shape=1/lung.wbs
$scale,lower.tail=FALSE),from=0, to=max(lung$time))
lines(survfit(Surv(time,status)~1, data=lung), col="red")
abline(h=0.5,lty=2)
abline(v=unique(predict(lung.wbs,type="quantile",p=0.5)),lty=2,col=4)
However, one last technical question. In an intercept only model, how
do you specify newdata to get just a single estimate? In the above
example I used unique against the model predictions evaluated at every
datapoint. But with no terms on the RHS of the model, how do you (can
you) specify new data?
Best wishes,
Andrew
On 26 Nov 2008, at 16:27, Andrew Beckerman wrote:
Dear all -
I have followed the thread the reply to which was lead by Thomas
Lumley about using pweibull to generate fitted survival curves for
survreg models.
http://tolstoy.newcastle.edu.au/R/help/04/11/7766.html
Using the lung data set,
data(lung)
lung.wbs <- survreg( Surv(time, status)~ 1, data=lung, dist='weibull')
curve(pweibull(x, scale=exp(coef(lung.wbs)), shape=1/lung.wbs
$scale,lower.tail=FALSE),from=0, to=max(lung$time))
lines(survfit(Surv(time,status)~1, data=lung), col="red")
Assuming this is correct, why does the inflection point of this
curve not match up to the exp(scale parameter)? Am I wrong in
assuming that the scale represents the inflection, and the shape
adjusts the shape around this point? I think I am.... perhaps
confusing the scale and the median with the inflection point
calcuation?
One can visualise the mismatch with:
abline(v=exp(coef(lung.wbs)),lty=2)
abline(h=0.5,lty=2)
Many thanks for the clarification....
R version 2.8.0 (2008-10-20)
i386-apple-darwin8.11.1
locale:
en_GB.UTF-8/en_GB.UTF-8/C/C/en_GB.UTF-8/en_GB.UTF-8
attached base packages:
[1] splines datasets utils stats graphics grDevices
methods base
other attached packages:
[1] survival_2.34-1 Hmisc_3.4-3 lattice_0.17-15 MASS_7.2-44
loaded via a namespace (and not attached):
[1] cluster_1.11.11 grid_2.8.0 tools_2.8.0
Andrew
---------------------------------------------------------------------------------
Dr. Andrew Beckerman
Department of Animal and Plant Sciences, University of Sheffield,
Alfred Denny Building, Western Bank, Sheffield S10 2TN, UK
ph +44 (0)114 222 0026; fx +44 (0)114 222 0002
http://www.beckslab.staff.shef.ac.uk/
http://www.flickr.com/photos/apbeckerman/
http://www.warblefly.co.uk
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