stepwise text

Phoenix Certara Forums Modelling and Simulation
1

Dear all,

I have a problem about the Stepwise run model. I built a POP PK model, after I finished covariates stepwise, everything seems to be good, but when I check the Results—TextOutput—Stepwise Text, it confuses me that the criterion of stepwise forward turn from 3.841 into 5.991 from cstep010 to cstep015（Add P value=0.05）? Does anyone know why?

Thanks a lot.

2

Hi Crystal,

This is the explanation for your question

• The LRT compares two likelihoods • For nested models the difference of two log likelihoods is asymptotically chi-squared distributed: – For 1 parameter difference (1 degree of freedom) the critical value from the chi-squared distribution is ‘3.84’ (P=0.05) \

http://holford.fmhs.auckland.ac.nz/docs/principles-of-covariate-modelling.pdf

The attached link could be of use to you.

Thanks

Chandramouli (Mouli)

3

Thanks, Chandramouli for your help, I really appreciate it.

4

HI Crystal,

Forgot to add the explanation - With RACE you might have 2 levels and the p value is 0.05

with

df =2 it is - 5.991465.

df=1 it is - 3.841459

Thanks

Mouli

5

you can use R to know what are the thresholds:

qchisq(1-0.05,1:5)
[1] 3.841459 5.991465 7.814728 9.487729 11.070498
qchisq(1-0.01,1:5)
[1] 6.634897 9.210340 11.344867 13.276704 15.086272
qchisq(1-0.001,1:5)
[1] 10.82757 13.81551 16.26624 18.46683 20.51501

the above cover p = 0.05, 0.01 and 0.001
for 1 to 5 degrees of freedom

Samer