[R] Negative binomial regression for count data

[vito muggeo]

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
> 
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> 
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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