Date: Wed, 30 Sep 2009 18:53:00 -0400
Reply-To: Wensui Liu <liuwensui@GMAIL.COM>
Sender: "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From: Wensui Liu <liuwensui@GMAIL.COM>
Subject: Re: Poisson
In-Reply-To: <e3b6b0c1-39d2-4d90-b345-c628c4414602@m38g2000yqd.googlegroups.com>
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you might want to take a look at hurdle model, since hurdle model is
able to take care of both over- and under-dispersion.
On Wed, Sep 30, 2009 at 3:33 PM, Shawn Haskell
<shawn.haskell@state.vt.us> wrote:
> On Sep 29, 8:00 pm, liuwen...@GMAIL.COM (Wensui Liu) wrote:
>> Derek,
>> You might want to take a look at my sugi paper in 2008. In my paper, I
>> provided several statistical tests for over-dispersion.
>>
>> wensui
>>
>>
>>
>>
>>
>> On Tue, Sep 29, 2009 at 10:40 AM, Reed, Derek <derek.r...@sinclair.edu> wrote:
>> > Morning Folks,
>>
>> > In performing a Poisson regression using GENMOD, does anyone have a
>> > metric for how far is too far away from one for the scale parameter? I
>> > am trying to decide when the data are too over dispersed. When I re-run
>> > an analysis using the negative binomial distribution my 1.26 scale
>> > parameter becomes .80. Any help/guidance would be appreciated. Many
>> > thanks!
>>
>> > Derek
>>
>> --
>> ==============================
>> WenSui Liu
>> Blog : statcompute.spaces.live.com
>> Tough Times Never Last. But Tough People Do. - Robert Schuller
>> ==============================- Hide quoted text -
>>
>> - Show quoted text -
>
> very good - thanks for that WenSui. What's the latest on under-
> dispersion if you can help there too? I have counts (response
> variable) of 0-4. It is not a ZIP situation, and deviance/df < 0.5.
> The scale parameter being <1 artificially reduces Type-3 SEs and p-
> values. So, I switched to an ordinal logistic model that seems to fit
> the data much better according to AICc. Any other thoughts? thanks.
> Shawn
>
--
==============================
WenSui Liu
Blog : statcompute.spaces.live.com
Tough Times Never Last. But Tough People Do. - Robert Schuller
==============================
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