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

 From "springathens@gmail.com" To statalist@hsphsun2.harvard.edu Subject Re: st: RE: Checking Goodness of Fit after Zero-inflated Poisson Date Sun, 22 May 2011 15:04:18 -0400

```I got it.
So,  Log likelihood and its corresponding chi-squared already tell the
goodness-of-fit.
Thanks for your clarification.

Sangyub

On Sun, May 22, 2011 at 2:56 PM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
> 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
>
> Thanks Nick for your response.
>
> First, sorry for not understanding the name policy.  I will sincerely
> follow the rules.
>
> 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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```