[R-sig-ME] longitudinal data/repeated measure

[ONKELINX, Thierry]

Dear Ludovico,

Use time as a factor.

Best regards,

Thierry

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

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

-----Oorspronkelijk bericht-----
Van: r-sig-mixed-models-bounces op r-project.org [mailto:r-sig-mixed-models-bounces op r-project.org] Namens Ludovico Frate
Verzonden: woensdag 3 april 2013 16:28
Aan: r-sig-mixed-models op r-project.org
Onderwerp: [R-sig-ME] longitudinal data/repeated measure

Hi there,I need advice in longitudinal data/repeated measure analysis. In particular, I have some difficults about the fixed effect of time in these type of studies. With only two repeated measures, taken on the same subject two times (2001 and 2012) and if I want to detect if the response variable is different between the two measures  (2001 and 2012), how should I treat the time effect? As a continuos variable or a grouping factor?The measure were taken in 2001 and in 2012. If I explicit time as a continuous variable the model (linear mixed model) doesn't reach the convergence, because in this case, the intercept represent the measurement at time 0. Perhaps I should assign the value of 0 and 11 respectively to 2001 and 2012, or treat time as a grouping factor (time a and time b)?Thanks in advanceLudovico

Ludovico
Frate

PhD student (University of Molise - Italy) Environmetrics Lab
http://www.distat.unimol.it/STAT/environmetrica/organico/collaboratori/ludovico-frate-1
Department of Biosciences and Territory - DiBT Università del Molise.
Contrada Fonte
Lappone,
86090 -  Pesche (IS)
ITALIA.
Cel: ++39
3333767557
Fax: ++39 (0874) 404123
E-mail ludovico.frate op unimol.it
ludovicofrate op hotmail.it

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