From: Greg Snow <Greg.Snow_at_imail.org>

Date: Thu, 20 Dec 2007 14:53:18 -0700

Date: Thu, 20 Dec 2007 14:53:18 -0700

The other one I should have mentioned:

5.1: Use the glm function with family = poisson. The counts are the y variable and the x variable is either 0/1 or a 2 level factor indicating which group the values come from. The p-value for the slope of x tests for a difference in the 2 groups.

5.3 ...

-- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.snow_at_imail.org (801) 408-8111Received on Thu 20 Dec 2007 - 21:57:15 GMT

> -----Original Message-----

> From: r-help-bounces_at_r-project.org> [mailto:r-help-bounces_at_r-project.org] On Behalf Of Greg Snow> Sent: Thursday, December 20, 2007 1:20 PM> To: Mark Gosink; r-help_at_r-project.org> Subject: Re: [R] comparing poisson distributions>> There are a few different options that you can try depending> on your problem and your preferences:>> 1. For large lambda the poisson can be approximated by a> normal, for large n (even for small lambda) the mean is> approximately normal due to the central limit theorem. So if> your lambda and n are large enough in combination then you> could just do a standard 2 sample t-test (t.test> function) and use the approximate p-value from there.>> 2. Fit 2 models by maximum likelihood, one in which both> lambdas are equal and one in which they can differ (fitdistr> from MASS may help, or look at optim and friends), then do a> likelihood ratio test on the differences (-2 * likelihood> diff is approx chisquared(1) under the null).>> 3. Do a permutation test: find the difference in the> means/medians/(other stat of interest) between the 2 samples,> then permute the samples randomly (create 2 samples of the> same sizes from the original data values, but with random> assignment as to which group a value goes into) and find the> same difference, repeate a bunch of times (like 1998) and> combine all the differences found into a vector, plot the> histogram of the values and look at where the original> difference fits into the distribution. The number of values> that are as or more extreeme than the original value is your p-value.>> 4. Create logical bins for values (e.g. 0-1, 2-3, 4-6, 7+)> and count how many from each group fall in each bin, use> prop.test or chisq.test to see if the proportions differ.>> 5. Probably some others that don't come to mind right now.>> Hope this helps,>> --> Gregory (Greg) L. Snow Ph.D.> Statistical Data Center> Intermountain Healthcare> greg.snow_at_imail.org> (801) 408-8111>>>> > -----Original Message-----> > From: r-help-bounces_at_r-project.org> > [mailto:r-help-bounces_at_r-project.org] On Behalf Of Mark Gosink> > Sent: Tuesday, December 18, 2007 12:31 PM> > To: r-help_at_r-project.org> > Subject: [R] comparing poisson distributions> >> > Hello all,> >> > I would like to compare two sets of count data> which form> > Poisson distributions. I'd like to generate some sort of p-value of> > the likely-hood that the distributions are the same. Thanks> in advance> > for your advice.> >> >> >> > Cheers,> >> > Mark> >> >> >> > Mark Gosink, Ph.D.> >> > Head of Computational Biology> > Scripps Florida> > 5353 Parkside Drive - RFA> > Jupiter, FL 33458> > tel: 561-799-8921> > fax: 561-799-8952> > gosink_at_scripps.edu> >> >> >> >> > [[alternative HTML version deleted]]> >> > ______________________________________________> > R-help_at_r-project.org mailing list> > https://stat.ethz.ch/mailman/listinfo/r-help> > PLEASE do read the posting guide> > http://www.R-project.org/posting-guide.html> > and provide commented, minimal, self-contained, reproducible code.> >>> ______________________________________________> R-help_at_r-project.org mailing list> https://stat.ethz.ch/mailman/listinfo/r-help> PLEASE do read the posting guide> http://www.R-project.org/posting-guide.html> and provide commented, minimal, self-contained, reproducible code.>

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