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 Universit?tsstr. 5 45141 Essen (DE) Tel.: +49 (0)201/183-3113 Fax: +49 (0)201/183-2179 ?
How do I account for Unbalanced Observation-per-group in Linear Mixed Effect Models?
2 messages · Alessandro Manfrin, Thierry Onkelinx
3 days later
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 INSTITUUT VOOR NATUUR- EN BOSONDERZOEK / RESEARCH INSTITUTE FOR NATURE AND FOREST Team Biometrie & Kwaliteitszorg / Team Biometrics & Quality Assurance thierry.onkelinx at inbo.be Havenlaan 88 bus 73, 1000 Brussel www.inbo.be /////////////////////////////////////////////////////////////////////////////////////////// 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 /////////////////////////////////////////////////////////////////////////////////////////// <https://www.inbo.be> 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
Universit?tsstr. 5
45141 Essen (DE)
Tel.: +49 (0)201/183-3113
Fax: +49 (0)201/183-2179
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