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st: Re: Testing for differences in ratios


From   Roger Newson <roger.newson@kcl.ac.uk>
To   statalist@hsphsun2.harvard.edu
Subject   st: Re: Testing for differences in ratios
Date   Mon, 22 Dec 2003 19:48:09 +0000

At 13:16 22/12/03 -0600, Craig Goodman wrote:
I was hoping someone could provide some guidance on whether there is a test in Stata for determining whether there is statistically significant difference in ratios. In my case, I am examining committee seats and the ratio of majority members to minority members and comparing that to the chamber as a whole. Any thoughts?
Usually, when comparing 2 ratios, we measure ratios between ratios, not differences between ratios. In your case, you seem to be looking for logistic regression, which in Stata is handled by the -logit- procedure. The odds in favour of being a minority member is defined as the probability of being a minority member divided by the probability of being a majority member. Logistic regression is used to derive confidence intervals for odds and their ratios. In your case, you might have a binary variable -minority-, which is 1 for minority members and 0 for majority members, and a variable -committee-, which is 1 for committee members and 0 for non-committee legislators. If you type

logit minority committee, or

then Stata will give you a confidence interval for the ratio between the odds of being minority for committee members and the odds of being minority for non-committee legislators. And if you typed

gene byte baseline=1
logit minority committee baseline, or noconst

then Stata will give you confidence intervals for the baseline odds of being a minority (assuming that you are a non-committee legislator) and also for the odds ratio as before.

I hope this helps.

Roger


--
Roger Newson
Lecturer in Medical Statistics
Department of Public Health Sciences
King's College London
5th Floor, Capital House
42 Weston Street
London SE1 3QD
United Kingdom

Tel: 020 7848 6648 International +44 20 7848 6648
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or 020 7848 6605 International +44 20 7848 6605
Email: roger.newson@kcl.ac.uk
Website: http://www.kcl-phs.org.uk/rogernewson

Opinions expressed are those of the author, not the institution.

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