[R-sig-ME] How do I account for Unbalanced Observation-per-group in Linear Mixed Effect Models?

Thierry Onkelinx thierry.onkelinx at inbo.be
Tue Mar 6 09:45:55 CET 2018

Dear Alessandro,

You don't have to account for the different number of observations. The
random effects structures takes into account that the observations are not
independent. A river with twice the number of  observations will have more
influence, but probably less than twice. How much will depend on the
variance of the random effect.

You might want to read Zuur et al (2009) Mixed Effects Models and
Extensions in Ecology with R

Best regards,

ir. Thierry Onkelinx
Statisticus / Statistician

Vlaamse Overheid / Government of Flanders
Team Biometrie & Kwaliteitszorg / Team Biometrics & Quality Assurance
thierry.onkelinx at inbo.be
Havenlaan 88 bus 73, 1000 Brussel

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to say
what the experiment died of. ~ Sir Ronald Aylmer Fisher
The plural of anecdote is not data. ~ Roger Brinner
The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of data.
~ John Tukey


2018-03-02 11:08 GMT+01:00 Alessandro Manfrin <manfrin at igb-berlin.de>:

> Dear all,
> I have a dataset of fish collected in different rivers over different
> years each of them sampled a different number of times during different
> projects .
> This different number of observations among rivers in some cases can be
> important:
> For example:
> River X = 1 project (1 observation=1 sampling x 1 year);
> River Y = 5 projects (15 observations= 1 sampling x 3 years x 5 projects);
> River Z= 15 projects (105 observations=1 sampling x 7 years x 15 projects);
> I want to calculate in the all region (so not interested in specific
> rivers) how the abundance is related to Years, Latitude, Altitude and
> Anthopic pressures (APindex). I thought to use the following model:
>  lme: Abu~Years+Latitude+Altitude+APindex + (1|river/project) + corrARMA
> (form = Years|River/project).
>  -What is the influence of RiverZ with its 105 observation compared to the
> other rivers which have less number of observations?
> -Am I accounting for this unbalanced observations in the random structure
> (1|river/project)?
> -Do I have to account in the model for the different number of
> observations with (weight=1/n observation for each project?)
>  Thank you
> Dr. Alessandro Manfrin
> University of Applied Sciences Trier, Umwelt Campus Birkenfeld/
> University of Duisburg-Essen, Faculty of Biology
> Working place:
> Department of Aquatic Ecology
> Room S05 T03 B02
> Universitätsstr. 5
> 45141 Essen (DE)
> Tel.: +49 (0)201/183-3113
> Fax: +49 (0)201/183-2179
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