[R-sig-ME] time-series analysis: how to deal with 'nuisance'factors influencing a trend?
davidD at qimr.edu.au
Thu Dec 16 07:44:41 CET 2010
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 \_,-._/
| 300 Herston Rd, Brisbane, Queensland 4029, Australia GPG 4D0B994A v
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