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Re: st: RE: zero-inflated analyses: when do you decide that is zero-inflated?
From
"Cris Dogaru (Oregon State University)" <[email protected]>
To
[email protected]
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
<[email protected]> 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: [email protected] [[email protected]] on behalf of Cris Dogaru (Oregon State University) [[email protected]]
> Sent: Monday, July 15, 2013 7:49 AM
> To: [email protected]
> 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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