[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


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


University of Patras , Greece                 

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