[R-sig-ME] zero inflated and repeat cells

[Highland Statistics Ltd]

>
> ------------------------------
>
> Message: 2
> Date: Tue, 10 Jul 2012 15:40:21 -0500
> From: "Stephanie L. Simek" <ssimek at CFR.MsState.Edu>
> To: <r-sig-mixed-models at r-project.org>
> Subject: [R-sig-ME] zero inflated and repeat cells
> Message-ID:
> 	<736BA42933F6C84698DBA828209ED41E01AD1026 at mail.cfr.msstate.edu>
> Content-Type: text/plain
>
> Dear R users,
>
>   
>
> I have count data that illustrates the presences or absence of
> individuals in my study population. I created a grid of cells across the
> study area and calculated a count value for each individual per season
> per year for each cell. The count value is the number of times an
> individual was present in each cell.
>
>   
>
> My data is set up with the cell ID repeated for each individual per
> season per year. This format results in 71,000 records (of which 925
> have a count value >0 and 70,075 have a count value = 0). I would like
> to run mixed effects model with individual and year as random effects
> but I have been advised two things:
>
>   

I don't know whether the approach sketched above is good, bad or 
pragmatic (I would need to
know more details, but I did raise my eyebrow),
but as to your specific questions below:

1. Not sure if I would agree. It all depends on where the zeros are, and 
whether
some of the covariates can be used to model the zeros....in which case 
even a Poisson GLM
may do the job. But a ZIP is probably ok.



2. You need to think very careful where any dependency structures are in 
your experiment.
Within a cell...between cells? Without further information I can't say.
Don;t you have spatial correlation between these cells?


Alain
> 1.      The data are too extremely zero inflated therefore a zero
> inflated Poisson model will not work.
>
> 2.      The model cannot be run because the grid cells are repeated for
> each individual per season per year. I am told the model doesn't
> recognize that Cell ID #1 for individual "A" is the same cell for Cell
> ID #1 for individual "B".
>
>   
>
> Does anyone know if either or both of these points are true? Also, does
> anyone have any recommendations.
>
>   
>
> Thank you,
>
>   
>
> -Stephanie
>
>   
>
>   
>
> -------------------------------------------------------
>
> Stephanie L. Simek
>
> Carnivore Ecology Lab
>
> Forest and Wildlife Research Center
>
> Mississippi State University
>
> Box 9690
>
> Mississippi State, MS 39762
>
> Cell: (850) 591-1430
>
> Email: ssimek at cfr.msstate.edu
>
>
>
>
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> End of R-sig-mixed-models Digest, Vol 67, Issue 13
> **************************************************
>


-- 

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


4. Zero Inflated Models and Generalized Linear Mixed Models with R. (2012) Zuur, Saveliev, Ieno.
http://www.highstat.com/book4.htm

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


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