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# How to calculate P value for Poisson distribution?

Can anybody please let me know how to calculate P value for a Poisson distribution? Thanks.

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Solution
‎06-09-2016 09:49 PM
New Contributor
Posts: 3

## Re: How to calculate P value for Poisson distribution?

Thank you so much. It is really helpful. Could you also please guide me to help finding appropritae sas code in such cases. Thanks a lot.

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Super User
Posts: 20,781

## Re: How to calculate P value for Poisson distribution?

I believe that is using the CDF, cumulative distribution function.

http://support.sas.com/documentation/cdl/en/lefunctionsref/63354/HTML/default/viewer.htm#n0n7cce4a3g...

Solution
‎06-09-2016 09:49 PM
New Contributor
Posts: 3

## Re: How to calculate P value for Poisson distribution?

Thank you so much. It is really helpful. Could you also please guide me to help finding appropritae sas code in such cases. Thanks a lot.

SAS Super FREQ
Posts: 3,842

## Re: How to calculate P value for Poisson distribution?

You accidentally marked your own replay as the solution.

SAS Super FREQ
Posts: 3,842

## Re: How to calculate P value for Poisson distribution?

If you have an observed value of some test statistics that follows a Poisson distribution, then Reeza is correct and you can use the CDF function:

lambda=10; obsValue=15;
OneSidePVal = 1 - cdf("Poisson", ObsValue, lambda);

However, since most statistics do not follow a Poisson distribution, maybe you are asking about fitting a Poisson distribution to data and obtaining a p-value for the goodness-of-fit test? In that case, you should use PROC GENMOD to fit the distribution as shown in the article "Fitting a Poisson distribution to data in SAS". (You can also use COUNTREG.)

Unfortunately, the statistics that come out of PROC GENMOD do not include p-values, but you can use PROC FREQ to compute a chi-square statistics that compares the observed and expected values in each category. The details are shown in the article "Testing the fit of a discrete distribution."  If you are interested, there are also graphical techniques for evaluating the fit.

New Contributor
Posts: 3

## Re: How to calculate P value for Poisson distribution?

Thank you so much. Really appreciate. Best regards.

☑ This topic is solved.