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Re: st: RE: zero-inflated analyses: when do you decide that is zero-inflated?


From   "Cris Dogaru (Oregon State University)" <statamplus@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: zero-inflated analyses: when do you decide that is zero-inflated?
Date   Tue, 16 Jul 2013 12:03:15 +0200

Thank you very much, Peter.
No, the data does not seem to come from a Poisson distribution (I
quickly tested it with a Java Applet
http://home.ubalt.edu/ntsbarsh/Business-stat/otherapplets/PoissonTest.htm).

And as David Hoaglin says, this does not qualify for a
Poisson/negative binomial distribution; conceptually my outcome is not
a count variable (counting events), but
rather a set of indicator variables for a latent construct (atopy or
sensitization).

I will use a two-part model as you suggest, but I will do it
individually for each of the 4 skin prick tests and use the actual
size in mm of the skin reaction (wheal).

All the best

Cris Dogaru






















On Mon, Jul 15, 2013 at 5:17 PM, Lachenbruch, Peter
<Peter.Lachenbruch@oregonstate.edu> wrote:
> My view has been if you can justify the zeros being identified you can use the two-part model.  If you can't argue that, then zinp or zinb would be used.  If the data look Poisson, then you can just use poisson.  The usual test of variance*(n-1) divided by mean squared.
>
> Peter A. Lachenbruch,
> Professor (retired)
> ________________________________________
> From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] on behalf of Cris Dogaru (Oregon State University) [statamplus@gmail.com]
> Sent: Monday, July 15, 2013 7:49 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: zero-inflated analyses: when do you decide that is zero-inflated?
>
> Dear Stata users,
>
> I couldn't find an answer to this apparently simple question: how does
> one decide that a distribution is zero-inflated, so that one can use
> zero-inflated Poisson regression or zero-inflated negative binomial
> regression?
>
> More concrete: my outcome variable is number of positive skin prick
> tests (done for 4 allergens, therefore the number has a range 0 to 4).
> Here are the summary tables; is this zero-inflated?..
>
>
> spt_number -- number of positive (wheal>3mm) STP
> -----------------------------------------------------------
>               |      Freq.    Percent      Valid       Cum.
> --------------+--------------------------------------------
> Valid   0     |        853      57.02      58.30      58.30
>           1     |        286      19.12      19.55      77.85
>           2     |        176      11.76      12.03      89.88
>           3     |        105       7.02       7.18      97.06
>           4     |         43        2.87       2.94     100.00
>          Total |       1463      97.79     100.00
> Missing .     |         33       2.21
> Total         |       1496     100.00
> -----------------------------------------------------------
>
> . fsum spt_number
>
>    Variable |        N     Mean       SD      Min      Max
> ------------+---------------------------------------------
>  spt_number |     1463     0.77     1.10     0.00     4.00
>
> Many thanks
> Cristian Dogaru
> ISPM, University of Bern
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