[R-sig-ME] time-series analysis: how to deal with 'nuisance'factors influencing a trend?

[David Duffy]

On Wed, 15 Dec 2010, Giancarlo Sadoti wrote:

> I'm examining longitudinal data of counts [COUNT] of a species across 
> three years [YEAR] and across 40 sites [SITE].  My primary interest is 
> in the coefficient of YEAR to determine if there is a trend across the 
> three years.  Counts fit a poisson distribution.  I can't cite this, but 
> it is my understanding that three years is inadequate for the inclusion 
> of a temporal correlation structure, so I am not including it.

As I understand it, 3 years is not enough data to _differentiate_ between 
an autoregressive model with innovations, and a "general" correlation 
model.  You can definitely include YEAR random effects and either model 
will fit equally well, and may or may not be a significant improvement on 
your original model.

Cheers, David Duffy.

-- 
| David Duffy (MBBS PhD)                                         ,-_|\
| email: davidD at qimr.edu.au  ph: INT+61+7+3362-0217 fax: -0101  /     *
| Epidemiology Unit, Queensland Institute of Medical Research   \_,-._/
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