Re: from odds to probs
From: Anon. (bob.ohara_at_NOSPAM.helsinki.fi)
Date: 01/17/05
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Date: Mon, 17 Jan 2005 08:39:03 +0200
Doug wrote:
> Hi,
>
> This simple enquiry is one that I haven't solved this morning and is
> based on the odds ratio for logistic regression and the associated
> probabilities.
>
> The odds ration equation is O=P/(1-P) (odds=prob/1-prob) of event.
> However in Paul Allison's logiistical regression book (page 11) he
> also has P=O/1+O. While I know that this is true and is easily
> checked with real data (eg 0.2=0.25/1+0.25) and (0.25=0.2/0.8), I
> haven't done this type of maths for a while.
>
> Could someone remind me of the maths steps in getting from O=P/1-P to
> P=O/1+O.
>
> It's monday - ok!
>
It is!
Write P/(1-P) as 1/(1/P-1), have a strong cup of coffee, and you'll find
it much easier.
Bob
-- Bob O'Hara Department of Mathematics and Statistics P.O. Box 68 (Gustaf Hällströmin katu 2b) FIN-00014 University of Helsinki Finland Telephone: +358-9-191 51479 Mobile: +358 50 599 0540 Fax: +358-9-191 51400 WWW: http://www.RNI.Helsinki.FI/~boh/ Journal of Negative Results - EEB: www.jnr-eeb.org
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