nested mixed model with covariate and missing data

Dear Carlos,

Use baseline as an offset factor. That is equivalent of subtracting the baseline from the activity prior to analysis.
(1|subject/neuron) gives you a random effect for both the subject level as the neuron within subject level.
Don't use (1|time) in combination with time as a fixed effect unless time is continuous and you have a fairly large number of different timepoints.
Missing data is not a problem as long as it is missing at random.

So your model looks like this
activity ~ offset(baseline) + treatment*time + (1|subject/neuron)

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 at inbo.be
www.inbo.be

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-----Oorspronkelijk bericht-----
Van: r-sig-mixed-models-bounces at r-project.org [mailto:r-sig-mixed-models-bounces at r-project.org] Namens Carlos Gias
Verzonden: woensdag 2 januari 2013 13:48
Aan: r-sig-mixed-models at r-project.org
Onderwerp: [R-sig-ME] nested mixed model with covariate and missing data

Hi,

I am new to mixed models and not sure how to design a model for my data to use with the lmer function.


I am trying to study the effect of a drug (treatment) in subjects along time. For every subject we measure activity from a (variable) number of neurons at different time points. Therefore, the neuron measurement is nested within subject. I would also like to use the baseline activity measurement as a covariate for each neuron. This is a small sample of the dataset:

subject treatment neuron baseline time activity
1 1 1 3.06 1 7.02
1 1 1 3.06 2 6
1 1 1 3.06 3 9
1 1 2 3 1 5
1 1 2 3 2 6
1 1 2 3 3 9
2 2 3 4.77 1 3
2 2 3 4.77 2 2
2 2 3 4.77 3 1
3 2 3 2.14 1 2.03
3 2 3 2.14 2 2
3 2 3 2.14 3 1.5

I was wondering if the following would be an appropriate model.

activity ~ baseline + treatment*time + (1|subject:neuron) + (1|time)


I am also wondering how I could deal with the problem of missing data. Is there a function/package that could be helpful in this design?

I hope you can help.

Best regards,

Carlos

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