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Re: Covariate modelling question

From: Kajsa Harling <kajsa.harling>
Date: Thu, 26 Feb 2015 11:28:56 +0100
Dear Fiona,

This sounds like a PsN question, but it is impossible to answer without the example output. The mailing list does not accept long messages or attachments, so I suggest you send the original email with all output directly to me (On 02/26/2015 11:00 AM, Fiona Vanobberghen wrote:
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

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 Vanobberghen (née Ewings), PhD
Swiss Tropical and Public Health Institute
Socinstrasse 57, 4051, Basel, Switzerland
Tel: +41 61 284 87 41

Kajsa Harling, PhD
System Developer
Department of Pharmaceutical Biosciences
Uppsala University
Received on Thu Feb 26 2015 - 05:28:56 EST

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