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Re: st: Re: adjusted percentages and adjusted odds ratios


From   Michel Camus <mcamus@videotron.ca>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Re: adjusted percentages and adjusted odds ratios
Date   Sat, 20 Sep 2003 11:07:30 -0400

May I suggest that you submit a bit more info to the list?
Can you say something about the model used to estimate the adjusted OR and the method used to adjust the percentages?
What variables were used, what main effect was estimated?
Are the adjusted percentages obtained from a different model or are they computed from results of "-predict" ?

A few things seem odd. The gap between the calculated OR (1.17) from the adjusted percentages and the adjusted OR (1.7) seems inexplicable, unless ...
1) the covariates modelled were not the same,
2) the samples used for the predictions are not the same (e.g.e(sample) used or not in predicting),
3) the effect measured is not the same (e.g. one case specifies "due to cost" and the other "for any reason"),
or 4) an interaction term is entered in one model, not in the other.

I don't understand the study design. How did you select your subjects?
Could you select patient who cut back medication vs. patients who did not cut back, OR who cut back for other reasons, and then considered their type of insurance? This approach would seem unlikely to me.
If instead you selected and compared VA vs. private insurance holders as to whether or not and why they cut back their meditation, then should you not be computing the RR instead of the OR?

Michel

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Michel Camus, Ph.D.

Epidemiologist, Biostatistics and Epidemiology Div., HECSB, Health Canada

Courriel / e-mail : mcamus@videotron.ca <mailto:mcamus@videotron.ca>

Téléphone / phone : (514) 850-0157

Télécopieur / fax : (514) 850-0836

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~





-----Original Message-----
From: Copeland, Laurel
Sent: Friday, September 19, 2003 11:26 AM
To: Eby, Elizabeth
Cc: VHAANN HSRD Statistical Group
Subject: RE: adjusted percentages


The raw odds ratio can be calculated...17% of 334=277, 20% of 2741=2193
cut back?
no yes
priv no 277 57
ins? yes 2193 548

OR = (277/57) / (2193/548)
= 4.86 / 4.00
= approx 1.22
The remainder of the effect in the adjusted OR must be due to the
covariates.

-----Original Message-----
From: Eby, Elizabeth
Sent: Friday, September 19, 2003 11:05 AM
To: VHAANN HSRD Statistical Group
Subject: adjusted percentages

How would you explain the small difference in the adjusted percentages

(17%


vs 20%) with an odds ratio of 1.7 (p=.04)? Does it have anything to do

with


the underlying sample sizes of the 2 groups (n=334 VA patients and n=2741
with private insurance)?

Thanks.

-----Original Message-----
From: jpiette@umich.edu [mailto:jpiette@umich.edu]
Sent: Friday, September 19, 2003 10:51 AM
To: elizabeth.eby@med.va.gov
Subject:

Liz - can you double check this? THe difference in %'s seem small

relative


to
the AOR.

Controlling for their sociodemographic characteristics, number of chronic
diseases, and number of prescription medications, 17% of VA patients cut
back
on medication use due to cost compared to 20 % of those with private
insurance
(adjusted odds ratio [AOR]: 1.7, p=.04),
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