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# RE: st: RE: Checking Goodness of Fit after Zero-inflated Poisson

 From Nick Cox To "'statalist@hsphsun2.harvard.edu'" Subject RE: st: RE: Checking Goodness of Fit after Zero-inflated Poisson Date Sun, 22 May 2011 19:56:54 +0100

```You shouldn't need to ask. Your question was already answered in my post.

As the example output shows, -zip- already shows a LR chi-square statistic and a P-value.

Nick
n.j.cox@durham.ac.uk

springathens@gmail.com

First, sorry for not understanding the name policy.  I will sincerely

Ok, after I run a standard Poisson, I can compute the goodness-of-fit
by using the command -estat gof-.  For instance, after I run Poisson,
-estat gof-  reports as follows:

. poisson  day windcat du50 du100 du150

Iteration 0:   log likelihood = -186.58213
Iteration 1:   log likelihood = -183.38329
Iteration 2:   log likelihood = -183.34036
Iteration 3:   log likelihood = -183.34036

Poisson regression                                Number of obs   =        125
LR chi2(4)      =     606.33
Prob > chi2     =     0.0000
Log likelihood = -183.34036                       Pseudo R2       =     0.6231

------------------------------------------------------------------------------
day |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
windcat |   .6731493   .0625124    10.77   0.000     .5506273    .7956713
du50 |   2.550244   .3132588     8.14   0.000     1.936268     3.16422
du100 |   2.144695   .3203497     6.69   0.000     1.516821    2.772568
du150 |    2.07342   .3292282     6.30   0.000     1.428144    2.718695
_cons |  -2.008924   .2934786    -6.85   0.000    -2.584132   -1.433716
------------------------------------------------------------------------------

. estat gof

Goodness-of-fit chi2  =  118.6753
Prob > chi2(120)      =    0.5170

But, that command does not work after zip.  How may I compute the
goodnes-of-fit? Or, does zip already report the goodness-of-fit in its
result table?

Hope this clarifies my question.

Thanks again,
Sangyub Ryu

On Sun, May 22, 2011 at 2:18 PM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
> Dear Spring [in?] Athens:
>
> Please note the request in the Statalist FAQ to use real names.
>
> From a -zip- I see output like this
>
> Zero-inflated Poisson regression                  Number of obs   =        250
>                                                  Nonzero obs     =        108
>                                                  Zero obs        =        142
>
> Inflation model = logit                           LR chi2(2)      =     506.48
> Log likelihood  = -850.7014                       Prob > chi2     =     0.0000
>
> ------------------------------------------------------------------------------
>       count |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
> -------------+----------------------------------------------------------------
> count        |
>     persons |   .8068853   .0453288    17.80   0.000     .7180424    .8957281
>    livebait |   1.757289   .2446082     7.18   0.000     1.277866    2.236713
>       _cons |  -2.178472   .2860289    -7.62   0.000    -2.739078   -1.617865
> -------------+----------------------------------------------------------------
> inflate      |
>       child |   1.602571   .2797719     5.73   0.000     1.054228    2.150913
>      camper |  -1.015698    .365259    -2.78   0.005    -1.731593   -.2998038
>       _cons |  -.4922872   .3114562    -1.58   0.114     -1.10273    .1181558
> ------------------------------------------------------------------------------
>
> What else, precisely, do you have in mind by "goodness of fit"?
>
> Nick
> n.j.cox@durham.ac.uk
>
> springathens@gmail.com
>
> I am using zero-inflated Poisson regression.
> Unlike a standard Poisson regression, neither  -estat gof- nor
> -poisgof- commands report the goodness of fit for a zero-inflated
> Poisson.
> How may I find the goodness of fit for zero-inflated Poisson regression?

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```