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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 13:42:29 +0100

The 'p' is the calculation from p=o/(1+o), and all the numbers were calculated, apart from the OR and the number of cases (218) and controls (14748). 

Taking 'unconsciousness' as the example:

P= 3.54/(1+3.54)
P= 0.77973

Then I multiplied by the number of cases first, then controls: 

0.77973 * 218 = 170 (case unexposed)
218 - 170 = 48 (case exposed)

0.77973 * 14748 = 11500 (control unexposed)
14748 - 11500 = 3248 (control exposed)

I'm not sure what you mean by the baseline odds ratio? Is that the reference group odds, which is 1? The paper presents the information as (OR, 95% CI), with the outcome being infection:

Unconsciousness:    3.54 (1.93 - 6.00)     
No unconsciousness: 1.00

Is that clearer? I'm not sure I've presented that as well as I could...Thanks!

Carina

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Maarten Buis
Sent: 31 May 2011 13:26
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: Recovering cell values from odds ratios

On Tue, May 31, 2011 at 2:06 PM, King, Carina wrote:
> 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!

I don't understand what you did your example. What is p and where did
it come from? Which part of those numbers did you compute and which
were given. How did you do the computations?  Where is the baseline
odds?

Generally, I think your example contains too many variables. The best
strategy for problems like these is to start with an easy example,
i.e. one variable, work till you understand that and than complicate
matters till you get what you want.

-- Maarten

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


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

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