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Re: st: Interpreting conditional logistic regression equations using 2 similar types of matching.


From   Ronán Conroy <rconroy@rcsi.ie>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Interpreting conditional logistic regression equations using 2 similar types of matching.
Date   Wed, 7 Feb 2007 10:28:22 +0000

On 6 Feabh 2007, at 23:40, Donald Spady wrote:

This may be a second posting. The first one did not seem to go through and
on reading the FAQ I realized that I might have sent non-plain text. (I
copied the output from Stata using the copy table command rather than the
copy text command. Hopefully this one will work.)

I am looking at the effect of iron-deficiency anemia on the likelihood of a
child having a seizure.
I'm not sure I understand this design. Children can have more than one seizure, so is your case variable whether the child ever had a seizure or never had a seizure, or are you doing a nested case- control and matching a child in whom a seizure occurred with a child of the same age in whom one did not occur?




I am using clogit but have a problem in that I have
matched my case/control "group" variable (age) as either very tightly
matched (same month:  "tight") or more loosely matched (up to 3 months
difference: "loose").
Age is a measure of exposure to risk. This, and the fact that cases are defined by the occurrence of an event, suggest survival models here rather than conditional logistic regression.



The whole age range of difference is 30 months, so 3
months is still fairly closely matched. When I do the 2 clogit equations I
get different Odds ratios (which seems OK), but don't know which one is the
'right' one to use. Any help would be appreciated.
You are losing cases (output omitted) with both of your current designs, which is going to lower your power and, I'm guessing, bias the estimates in ways you can't predict but have certainly demonstrated.

I'd opt for a strategy that didn't lose information, starting maybe with a survival model approach.

=========
Ronán Conroy
Royal College of Surgeons in Ireland
rconroy@rcsi.ie
+353 (0) 1 402 2431
+353 (0) 87 799 97 95
http://www.flickr.com/photos/ronanconroy




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