Turn on suggestions

Auto-suggest helps you quickly narrow down your search results by suggesting possible matches as you type.

Showing results for

- Home
- /
- Analytics
- /
- Stat Procs
- /
- Fitting Overdispersed Poisson by MLE vs. MQLE

Options

- RSS Feed
- Mark Topic as New
- Mark Topic as Read
- Float this Topic for Current User
- Bookmark
- Subscribe
- Mute
- Printer Friendly Page

- Mark as New
- Bookmark
- Subscribe
- Mute
- RSS Feed
- Permalink
- Report Inappropriate Content

Posted 07-21-2014 11:31 AM
(1552 views)

Hi!

If I understand correctly, PROC GENMOD fits overdispersed Poisson models by maximum quasi-likelihood estimation (Generalized Linear Models Theory :: SAS/STAT(R) 12.1 User's Guide); they claim that the regression parameter estimates are not affected by the value of the dispersion / scale parameter, so I'm not really sure what role the dispersion / scale parameter plays in the quasi-likelihood function. It looks like there are four possible ways of incorporating the dispersion / scale parameter, either fit it or fix it one of three ways. You can fix it to a number, you can fix it to the chi-square divided by the degrees of freedom, or you can fix it to the deviance divided by the degrees of freedom. In each of these cases, it appears that "

It is known that if *Y* is overdispersed Poisson with scale parameter *Gamma*, then *(Y / Gamma)* has a Poisson distribution (http://fa319.voila.net/Schmidt.pdf). So it would be possible in theory to do a full MLE fit of the Poisson regression and the scale parameter together, but this would require keeping the factorial denominator of the Poisson formula in the likelihood function, and actually using its derivative. I wonder whether anyone has had any experience trying to fit an overdispersed Poisson by direct MLE this way?

Thanks!

2 REPLIES 2

- Mark as New
- Bookmark
- Subscribe
- Mute
- RSS Feed
- Permalink
- Report Inappropriate Content

Maybe you should check proc genmod .

- Mark as New
- Bookmark
- Subscribe
- Mute
- RSS Feed
- Permalink
- Report Inappropriate Content

I think the closest you could do in GENMOD is to iteratively estimate the SCALE parameter, feeding it back in to successive runs until a stable estimate was obtained.

But the easy way would be to use GLIMMIX. Look at example 43.12 Fitting a Marginal (GEE-Type) Model, for a method that directly accommodates overdispersion. Granted, the fit is through pseudo-likelihood, but that is the price you have to pay to get the extra scale parameter estimated directly.

Steve Denham

**Available on demand!**

Missed SAS Innovate Las Vegas? Watch all the action for free! View the keynotes, general sessions and 22 breakouts on demand.

What is ANOVA?

ANOVA, or Analysis Of Variance, is used to compare the averages or means of two or more populations to better understand how they differ. Watch this tutorial for more.

Find more tutorials on the SAS Users YouTube channel.