[R] survival parametric question

From: denis lalountas <dlalountas_at_yahoo.com>
Date: Sat 27 Aug 2005 - 16:50:59 EST


Hi to all,
I am working on design package using survival function. First using PSM and adopting a weibull specification for the baseline hazard , I have got the following results(since weibull has both PH and AFT propreties ,in addition I have used the PPHSm command):

  Value Std. Error z p

(Intercept) 1.768 1.0007 1.77 7.73e-02

SIZE -0.707 0.0895 -7.90 2.80e-15

REtoTA -0.896 0.4208 -2.13 3.33e-02

D1toEQ 0.281 0.0330 8.51 1.81e-17

EBTtoTA -6.706 1.0807 -6.21 5.46e-10

SALtoTA -3.943 0.3575 -11.03 2.78e-28

fishes 2.619 0.4194 6.24 4.26e-10

computers 2.781 0.2105 13.21 7.35e-40

Log(scale) -0.945 0.1514 -6.24 4.25e-10

and the loglikelihood -82.0

I dont know the specification of the weibull that Desing package uses so I can't evaluate the result.

For comparison reasons I have estimated the same model using another spftware EasyReg

wich gave the following results( the weibull specification has the form a(1).a(2).t^(a(2)-1):

parameters ML estimate t-value p-value Covariates

beta(1) 2.411460 2.136 0.03265 fishes

beta(2) 2.710115 3.322 0.00089 computers

beta(3) -7.539632 -2.646 0.00815 EBTtoTA

beta(4) -3.720231 -2.547 0.01086 SALtoTA

beta(5) 0.262115 1.982 0.04751 D1toEQ

beta(6) -0.710535 -0.515 0.60684 REtoTA

beta(7) -0.493369 -1.938 0.05262 LOG(SIZE)

alpha(1) 0.485828 0.392 0.69491

alpha(2) 2.597073 5.516 0.00000

log(L)=-83,4

First observe that the results are almost the same but the weibull parameters are not.

acooring to the weibull specification that easyreg uses a(2)>0 so the baseline hazard is monotonically increases ,acording to my expectations :(the empirical uncoditional hazard increases monotonically from t=1,12 and then decreases to zero)

My question is what is the weibull specification that R-design package uses for the baseline hazard. Second ,it is possible to plot the baseline hazard in R , in order to "see" the accelerating-decelerating effect in the AFT case.

In addition how can simulate a model in the AFT case ( some examples of simulation are given in the design manual for the COX-PH case.

I hope that my questions are not borring, if so sory I am a new user of R package.

Best regards

D.Lalountas

University of Patras , Greece                 


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