From: vito muggeo <vmuggeo_at_dssm.unipa.it>

Date: Fri 04 Mar 2005 - 00:50:40 EST

Date: Fri 04 Mar 2005 - 00:50:40 EST

Hi,

I do not know the article. Notice that an excess of zeroes can lead to
(spurious) overdispersion in data, therefore you should decide whether
assuming a zip ( zero excess coming from a "mixture") or a negBin (zero
execess due to overdispersion) model. Of course some likelihood based
criteria (eg AIC) could be of help for you

However it is possible (at least in principle) to account for both extra zeros and overdispersion as well. S. Jackman has written code to fit zip or zinb regression models http://pscl.stanford.edu/zeroinfl.r You should modify the code if you want to assume different "linear predictors" in the logit (zero vs non/zero) and count part

Hope this helps,

vito

Seyed Reza Jafarzadeh wrote:

> Dear list,

*>
**> I would like to fit a negative binomial regression model as described in "Byers AL, Allore H, Gill TM, Peduzzi PN., Application of negative binomial modeling for discrete outcomes: a case study in aging research. J Clin Epidemiol. 2003 Jun;56(6):559-64" to my data in which the response is count data. There are also 10 predictors that are count data, and I have also 3 categorical and 2 continious predictors. There is overdispersion in the distribution of the response variable (mean=2.8, variance=28) as well as in predictors, and there are a lot of zero's (zero-inflated).
**> The authors of that paper used PROC GENMOD in SAS 8.1. I wonder which of the following packages and tests to use in R to acheive such model for my analysis. Is there any tutorial available?
**>
**>
**> anova.negbin
**> Likelihood Ratio Tests for Negative Binomial GLMs
**> glm.convert
**> Change a Negative Binomial fit to a GLM fit
**> glm.nb
**> Fit a Negative Binomial Generalized Linear Model
**> negative.binomial
**> Family function for Negative Binomial GLMs
**> rnegbin
**> Simulate Negative Binomial Variates
**> theta.md
**> Estimate theta of the Negative Binomial
**> gam.neg.bin
**> GAMs with the negative binomial distribution
**> dnb2
**> Density for negative binomial, used in mmlcr
**> theta.mmmod
**> Estimate theta of the Negative Binomial by Moments
**> NegBinomial
**> The Negative Binomial Distribution
**> ezinb
**> The expected value of the censored zero-inflated negative binomial model
**>
**>
**>
**> Thanks,
**>
**> Reza
**>
**> [[alternative HTML version deleted]]
**>
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*

-- ==================================== Vito M.R. Muggeo Dip.to Sc Statist e Matem `Vianelli' Università di Palermo viale delle Scienze, edificio 13 90121 Palermo - ITALY tel: 091 6626240 fax: 091 485726/485612 ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.htmlReceived on Fri Mar 04 09:34:08 2005

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