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

Alessandro Manfrin manfrin at igb-berlin.de
Fri Mar 2 11:08:32 CET 2018

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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