[R-sig-eco] R-sig-ecology Digest, Vol 48, Issue 8

[Highland Statistics Ltd]

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> Dear all!
>           I have a question that needs help! In a zero-inflated model,  two explanatory variables are included to the count data, through the zeroinfl function, and the lrtest, I want to know the significance of each variable, but I don’t know how can I do this.
> the full mode is
> m1<-zeroinfl (count~A*B|A*B, dist="negbin", link="logit", data=mydata)
> How can I set another model that using the lrtest I can get the Chi-square, df, and P value of factor A ?
> There is an example in the book (Mixed effects models and extensions in ecology with R), but I can't get the results in P284 (Like Year (X2 = 12.553, df = 2, p = 0.002), Area (X2 = 47.599, df = 3, p<  0.001))Â .
> Can anyone help me? Thanks very much and best wishes!
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You need to dig into the zeroinfl code and figure out how to set the 
parameter k to a fixed value. In fact...the code uses log(k)....where k 
is the parameter in the variance term of the NB. Set the k of the 
submodels to the k of the full model. Then calculate the likelihood 
values of the submodels and apply the likelihood ratio test.

The drop1 function for a NB GLM does this all automatically...but for 
zeroinfl you have to do it manually.
A detailed example plus all R code is given in: Zero Inflated Models and 
Generalized Linear Mixed Models with R. Zuur, Saveliev, Ieno. (2012). It 
comes out in 2 weeks and can be ordered from 
http://www.highstat.com/book4.htm


Alain








-- 

Dr. Alain F. Zuur
First author of:

1. Analysing Ecological Data (2007).
Zuur, AF, Ieno, EN and Smith, GM. Springer. 680 p.
URL: www.springer.com/0-387-45967-7


2. Mixed effects models and extensions in ecology with R. (2009).
Zuur, AF, Ieno, EN, Walker, N, Saveliev, AA, and Smith, GM. Springer.
http://www.springer.com/life+sci/ecology/book/978-0-387-87457-9


3. A Beginner's Guide to R (2009).
Zuur, AF, Ieno, EN, Meesters, EHWG. Springer
http://www.springer.com/statistics/computational/book/978-0-387-93836-3


Other books: http://www.highstat.com/books.htm


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