It's difficult to interpret this without seeing the univariate and
multivariate results for both variables.
______________________________________________
Kieran McCaul MPH PhD
WA Centre for Health & Ageing (M573)
University of Western Australia
Level 6, Ainslie House
48 Murray St
Perth 6000
Phone: (08) 9224-2140
Fax: (08) 9224 8009
email: [email protected]
http://myprofile.cos.com/mccaul
http://www.researcherid.com/rid/B-8751-2008
______________________________________________
The fact that no one understands you doesn't make you an artist.
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Shell makka
Sent: Saturday, 14 February 2009 6:54 PM
To: [email protected]
Subject: Re: st: Overfitting?
Just for making it more clear in the cox model, its hazards ratio in
univariate was like:
categories HR CI P whole p
1 0.67 (0.56 - 0.82) <0.001
2 0 .51 (0.4 - 0.67) <0.001
3 0.36 (0.22 - 0.6) <0.001 <0.001
and in the multivariate model is like:
1 0.78 (0.63 -0.97) 0.03
2 0.82 (0.56 - 1.21) 0.32
3 0.94 (0.46 - 1.92) 0.88 0.13
So in the multivariate model the hazard is growing as year is
increasing (year categories:1, 2, 3)
Many thanks,
[Redacted]
On Sat, Feb 14, 2009 at 8:26 PM, Shell makka <[email protected]>
wrote:
> Thanks Maarten for your response, I will only use one of these
> covariates in my final model.
>
>
>
>
>
> On Sat, Feb 14, 2009 at 7:59 PM, Maarten buis
<[email protected]> wrote:
>> --- On Sat, 14/2/09, Shell makka wrote:
>>> Would you please let me know that when effect of one
>>> covariate flips in the multivariate model, is it only
>>> because of overfitting or it can have other reasons.
>>> This variable was significant in the univariate
>>> analysis but when I used it in multivariate model which
>>> contained another covariate that these too could be
>>> also colinear (one is calander year and the other is
>>> year of starting the study) it became insignificant and
>>> its effect reversed.
>>
>> I would not call this overfitting, it just means that most
>> of the effect of calender year occurs through year of
>> starting the study. However, you do need a good theoretical
>> reason for including both variables though. It is possible
>> that such a good theoretical argument exist, but it is
>> definitely not automatic.
>>
>> -- Maarten
>>
>> -----------------------------------------
>> Maarten L. Buis
>> Department of Social Research Methodology
>> Vrije Universiteit Amsterdam
>> Boelelaan 1081
>> 1081 HV Amsterdam
>> The Netherlands
>>
>> visiting address:
>> Buitenveldertselaan 3 (Metropolitan), room N515
>>
>> +31 20 5986715
>>
>> http://home.fsw.vu.nl/m.buis/
>> -----------------------------------------
>>
>>
>>
>>
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>>
>
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