Hi,
I'm fitting a Weibull model to interval (and right) censored survival
data
using survreg in Splus 5.1. I'm trying to use the "strata" argument in the
model formula as described in Venables and Ripley, 3rd edition, pages 376.
However, including strata(plot) in my model formula does not seem to result
in the fitting of different scale parameters for each level of "plot" and
does not seem to be any different from just including "plot" in the formula
without the strata argument. Thanks for any help with understanding this.
Two more related questions. Are there any Splus functions for fitting
a
Cox proportional hazard model to interval censored data or for estimating
survival curves for interval censored data?
Thanks,
Brian
options(contrasts=c("contr.treatment", "contr.poly"))
tmp<-Surv(Surv.dat.factor$I1, Surv.dat.factor$I2, type="interval2")
###### fit with strata argument
fit1a<-survreg(tmp~Species+Trt+Gap+strata(Plot), data = Surv.dat.factor,
link = c("log"),
dist = c("extreme"))
> fit1a
Call:
survreg(formula = tmp
~ Species + Trt + Gap + strata(Plot), data =
Surv.dat.factor, link
= c("log"), dist = c("extreme"))
Coefficients:
(Intercept) Specieslitu
Speciesquru TrtR TrtT+R TrtT TrtF
6.079439 -1.61673
1.156143 0.3354253 1.013128 1.040709 0.7447958
Gap
strata(Plot)Plot=gap12 strata(Plot)Plot=gap13
-0.6008336
0.5120529 0.4129829
strata(Plot)Plot=gap11
strata(Plot)Plot=gap1 strata(Plot)Plot=gap3
0.376538
0.4910114 0.2350804
strata(Plot)Plot=gap5
strata(Plot)Plot=gap6 strata(Plot)Plot=gap15
-0.156611
0.2999407 0.7962731
strata(Plot)Plot=gap7
strata(Plot)Plot=gap8 strata(Plot)Plot=gap10
0.4850185
0.788601 -0.9218784
Dispersion (scale) =
1.477747
Degrees of Freedom: 1080 Total; 1060 Residual
-2*Log-Likelihood:
3103.197
###### fit without strata
argument
fit1b<-survreg(tmp~Species+Trt+Gap+Plot, data = Surv.dat.factor,
link = c("log"),
dist = c("extreme"))
> fit1b
Call:
survreg(formula = tmp ~ Species + Trt + Gap + Plot, data =
Surv.dat.factor,
link = c("log"), dist =
c("extreme"))
Coefficients:
(Intercept) Specieslitu Speciesquru TrtR
TrtT+R TrtT TrtF
6.079439 -1.61673 1.156143 0.3354253
1.013128 1.040709 0.7447958
Gap Plotgap12 Plotgap13 Plotgap11
Plotgap1 Plotgap3 Plotgap5
-0.6008336 0.5120529 0.4129829 0.376538
0.4910114 0.2350804 -0.156611
Plotgap6 Plotgap15 Plotgap7 Plotgap8
Plotgap10
0.2999407 0.7962731 0.4850185 0.788601 -0.9218784
Dispersion
(scale) = 1.477747
Degrees of Freedom: 1080 Total; 1060
Residual
-2*Log-Likelihood: 3103.197
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