[R-sig-eco] Low counts

[Maarten de Groot]

Hi Tore,

How does your count distribution look like? Doe you have more zero's 
than expected (use zero inflated models), no zero's (use zero truncated 
models) or is there no problem with zero's? If it is the latter, it 
might be useful to try the negative binomial models (glm.nb()). Zuur et 
al (2009) gives a nice example that they still find a pattern in the 
residuals with a quasi poison model but no pattern with a negative 
binomial model.

Kind regards,

Maarten

Miltinho Astronauta wrote:
> Hi Tore,
>
> I put my 2cents on Zuur et al 2009's book - Mixed effect models...
> See Zero-Inflated examples in there.
>
> cheers
>
> milton
>
> 2010/2/1 Tore Chr Michaelsen <tore.michaelsen at bio.uib.no>
>
>   
>> Dear members;
>>
>> 1) I have fitted a glm to count data (using quasipoisson to correct for
>> disp.). In the final model, the relationship between Res and Fitted (i.e.
>> the line going through the plot) and QQ looks fine, but I am worried that
>> low count (one to five) could violate some assumption of the glm/poisson:
>> Although the line in the Res vs Fitted plot looks nice, the values show a
>> clear pattern (five diagonal lines = the counts). Crawley/R book says it
>> should look like the sky at night with no patterns. I assume patterns are
>> not visible with large counts (e.g. 0-100), but highly visible with low
>> counts as in this case. I still assume this is reason for some concern
>> about
>> the model, or is the concern not justified?
>>
>> 2) Any recommendations on literature regarding model inspection in R.
>>
>> Thank you for reading this mail!
>>
>> Best wishes;
>> Tore
>>
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