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Re: st: RE: probability models

From   Roger Newson <[email protected]>
To   [email protected]
Subject   Re: st: RE: probability models
Date   Mon, 07 Mar 2005 21:28:08 +0000

At 12:35 07/03/2005, Nick Cox wrote (in reply to Blau Blau):
For the normal, you have -ttest- and -ttesti-.

A comparison of two Poisson means is
possible using -poisson-. I am not
aware of an immediate equivalent.

[email protected]
If you are really confident that the 2 Poisson variables you are comparing really are Poisson, then you can use the fact that the conditional distribution of the first one, given their sum, is binomial, with total equal to their sum, and probability equal to


where mu_1 and mu_2 are the first and second Poisson population means. Therefore, you can use -cii- to define a binomial confidence interval for mu_1/(mu_1+mu_2), which should include 0.5 if mu_1 is equal to mu_2.

I hope this helps.


Roger Newson
Lecturer in Medical Statistics
Department of Public Health Sciences
Division of Asthma, Allergy and Lung Biology
King's College London

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United Kingdom

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Email: [email protected]

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

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