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st: Re: pprob


From   Stephen Sullivan <stephen_sullivan@brown.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: Re: pprob
Date   Sat, 2 Oct 2010 09:33:13 -0400

Hello.
Could someone please provide some Stata advice with respect to "pprob"?
I have a dataset, which does not appear to fit the Poisson distribution.
I then ran glm and nbreg commands as suggested (http://www.ats.ucla.edu/stat/stata/library/count.htm).
To see how the my variable fit the Poisson and negative binomial distributions, I downloaded and used the nbvargr command.
Nevertheless, something seems strange as all pprob values for the Poisson are 0.

The output is:
. nbvargr total

Obtaining Parameter Estimates

(9 observations deleted)
here

 Negative Binomial Probabilities
 with mean = 47.92857 & overdispersion = .0867781

    +------------------------------+
    |  k       nbprob        nbcum |
    |------------------------------|
 1. |  0   0.00000001   0.00000001 |
 2. |  1   0.00000006   0.00000006 |
 3. |  2   0.00000029   0.00000035 |
 4. |  3   0.00000105   0.00000140 |
 5. |  4   0.00000307   0.00000446 |
    |------------------------------|
 6. |  5   0.00000767   0.00001214 |
 7. |  6   0.00001703   0.00002917 |
 8. |  7   0.00003438   0.00006355 |
 9. |  8   0.00006417   0.00012772 |
10. |  9   0.00011222   0.00023994 |
    |------------------------------|
11. | 10   0.00018568   0.00042562 |
    +------------------------------+
k was byte now float

Poisson Probabilities for lambda = 47.92857

    +------------------------------+
    |  k        pprob         pcum |
    |------------------------------|
 1. |  0   0.00000000   0.00000000 |
 2. |  1   0.00000000   0.00000000 |
 3. |  2   0.00000000   0.00000000 |
 4. |  3   0.00000000   0.00000000 |
 5. |  4   0.00000000   0.00000000 |
    |------------------------------|
 6. |  5   0.00000000   0.00000000 |
 7. |  6   0.00000000   0.00000000 |
 8. |  7   0.00000000   0.00000000 |
 9. |  8   0.00000000   0.00000000 |
10. |  9   0.00000000   0.00000000 |
    |------------------------------|
11. | 10   0.00000000   0.00000000 |
    +------------------------------+
(11 observations deleted)

Any thoughts on what I may have done wrong?
Why all the zeros for the poisson prob?
Do I need to somehow install pprob?
Thank you so much for any help you can provide.

Stephen
Stephen R. Sullivan, MD, MPH
Assistant Professor, Surgery and Pediatrics
Brown University
Plastic Surgery
Rhode Island and Hasbro Children's Hospitals
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