[R-sig-ME] Model specification (fixed and random effects)?

[ONKELINX, Thierry]

Dear Julia,

Your model specification is OK if you assume that there is a linair trend along number of surgeries and tests.

The correlation should be  corAR1(form=~test_time|id/surgery_number)

Best regards,

ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature and Forest
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht
Belgium
+ 32 2 525 02 51
+ 32 54 43 61 85
Thierry.Onkelinx op inbo.be
www.inbo.be

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-----Oorspronkelijk bericht-----
Van: r-sig-mixed-models-bounces op r-project.org [mailto:r-sig-mixed-models-bounces op r-project.org] Namens Yla Savh
Verzonden: dinsdag 22 april 2014 9:07
Aan: r-sig-mixed-models op r-project.org
Onderwerp: [R-sig-ME] Model specification (fixed and random effects)?

Hello!

I would appreciate any help with my model design. Each subject had a few surgeries (from 1 to 5) ("surgery number"). These subjects had a test taken a few times after each surgery (from 1 to 6 times) ("test time"). There was supposed to be a decrease in the dependent variable (test_results) immediately after each surgery.  I coded both variables as continuous (surgery_number and test_time).

I wonder if I specified fixed and random effects correctly. I also think there might be some correlation between the surgery numbers but I am not sure about how to specify it in the model.

x<-lme(test_results~ surgery_number*test_time, random=~1|id/surgery_number) s12<-update(x, correlation=corAR1(form=~1|id/surgery_number))

Thanks!
Julia

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