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From: Fiona Vanobberghen <fiona.vanobberghen_at_unibas.ch>

Date: Thu, 26 Feb 2015 10:00:52 +0000

I posted this message a few days ago but it doesn't seem to have been

sent to the list - so I'm resending without the example output.

Best wishes

Fiona

--

Dear all

I am attempting to do some covariate modelling, using the scm wizard in

Pirana. I have seen some results which I wasn't expecting and would be

grateful if anyone could shed any light on it for me.

Initially, I used a forward inclusion p value of 0.1 and a backward

elimination p value of 0.05. This resulted in quite a complex

(implausible) model (we do have a reasonably large dataset), and I

decided to be more stringent, using p<0.05 for inclusion (and the same

p>0.05 for elimination at the last step). As a shortcut, I could see

from the output from the first attempt (with p<0.1) what I expected the

final model to look like if I were to run it again with p<0.05, ie where

the process would truncate. Just to double check (and verify that

nothing would be eliminated at the last step), I re-ran the scm wizard

with the more stringent p<0.05. And the results are not what I

expected... Below I have pasted the output for the first few forward

steps from each attempt. The results are essentially the same up until

the third step, although we see some small differences in the OFV

creeping in from the second step. However, at the fourth step, the

results are completely different. This isn't what I was expecting, based

on my understanding of the model selection process. Is this a known

behaviour? Has anyone experienced this problem and/or know why these

differences might occur? I'd be grateful for any advice.

Many thanks in advance for your help.

Best wishes

Fiona

--

*Fiona Vanobberghen (née Ewings), PhD*

Swiss Tropical and Public Health Institute

Socinstrasse 57, 4051, Basel, Switzerland

Tel: +41 61 284 87 41

Received on Thu Feb 26 2015 - 05:00:52 EST

Date: Thu, 26 Feb 2015 10:00:52 +0000

I posted this message a few days ago but it doesn't seem to have been

sent to the list - so I'm resending without the example output.

Best wishes

Fiona

--

Dear all

I am attempting to do some covariate modelling, using the scm wizard in

Pirana. I have seen some results which I wasn't expecting and would be

grateful if anyone could shed any light on it for me.

Initially, I used a forward inclusion p value of 0.1 and a backward

elimination p value of 0.05. This resulted in quite a complex

(implausible) model (we do have a reasonably large dataset), and I

decided to be more stringent, using p<0.05 for inclusion (and the same

p>0.05 for elimination at the last step). As a shortcut, I could see

from the output from the first attempt (with p<0.1) what I expected the

final model to look like if I were to run it again with p<0.05, ie where

the process would truncate. Just to double check (and verify that

nothing would be eliminated at the last step), I re-ran the scm wizard

with the more stringent p<0.05. And the results are not what I

expected... Below I have pasted the output for the first few forward

steps from each attempt. The results are essentially the same up until

the third step, although we see some small differences in the OFV

creeping in from the second step. However, at the fourth step, the

results are completely different. This isn't what I was expecting, based

on my understanding of the model selection process. Is this a known

behaviour? Has anyone experienced this problem and/or know why these

differences might occur? I'd be grateful for any advice.

Many thanks in advance for your help.

Best wishes

Fiona

--

*Fiona Vanobberghen (née Ewings), PhD*

Swiss Tropical and Public Health Institute

Socinstrasse 57, 4051, Basel, Switzerland

Tel: +41 61 284 87 41

Received on Thu Feb 26 2015 - 05:00:52 EST