[R] comparing poisson distributions

[Greg Snow]

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.

5.2 if this is just to make someone happy who always wants a p-value,
but doesn't understand it and will never actually use it, then use
runif.

5.3 ...

-- 
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 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]]
> > 
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> > 
> 
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