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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