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RE: st: Recovering cell values from odds ratios


From   "King, Carina" <c.king09@imperial.ac.uk>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Recovering cell values from odds ratios
Date   Tue, 31 May 2011 13:06:57 +0100

Thanks for replying, but I think I'm a bit confused, I've attached the example I'm using:

Cases: 218
Controls: 14748
Total: 14966

Risk factor:      OR:    p:       case exposed(a): control exposed (b): case unexposed(c): control unexposed(d):

Unconsciousness   3.54   0.77973   48		   3248			170			  11500
Age >65 years     1.69   0.62825   81 		   5483			137			  9265
Female gender     1.66   0.62406   82		   5544			136			  9204 

With the method you describe, the p for each comes out as shown above and consequently the numbers in each group. BUT when I then checked by recalculating the odds ratio from the numbers given, they all equal 1. What am I missing?  Thank you!

Carina

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Maarten Buis
Sent: 31 May 2011 12:25
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: Recovering cell values from odds ratios

On Tue, May 31, 2011 at 1:06 PM, King, Carina <c.king09@imperial.ac.uk> wrote:
> I am trying to convert odds ratios back to the raw numbers. I have information on the number of cases and number of controls, as well as the odds ratios and confidence intervals.

For that you also need the baseline odds, i.e. the odds of "success"
(the expected number of successes per failure) for the controls. The
odds for the cases are the odds ratio times the baseline odds. You can
transform those odds to probabilities using the formula p = o/(1+o),
where p is probability and o = odds. From there you can compute your
counts by multiplying the probabilities with the number of controls or
cases.

Hope this helps,
Maarten

--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany


http://www.maartenbuis.nl
--------------------------
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