[R-sig-ME] paired and repeated measurements

[ONKELINX, Thierry]

Dear Sven,

If you expect serial autocorrelation along time, then you could add a correlation structure like e.g. corAR1() or corExp(). You can find more details in Pinheiro & Bates (2000) or Zuur et al. (2009)

lme(co2~treatment+time, random=~1|site, correlation = corAR1(form = ~time), data=CO2_t1_t15)

@BOOK{
  title = {Mixed-Effects Models in {S} and {S-Plus}},
  publisher = {Springer},
  year = {2000},
  author = {Pinheiro, Jose C. and Bates, Douglas M.},
  note = {{ISBN 0-387-98957-0}},
  abstract = {A comprehensive guide to the use of the `nlme' package for linear
	and nonlinear mixed-effects models.},
  publisherurl = {http://www.springeronline.com/sgw/cda/frontpage/0,11855,4-10129-22-2102822-0,00.html?changeHeader=true}
}

@BOOK{
  title = {Mixed Effects Models and Extensions in Ecology with R},
  publisher = {Springer New York},
  year = {2009},
  author = {Zuur, Alain F. and Ieno, Elena N. and Walker, Neil J. and Saveliev,
	Anatoly A. and Smith, Graham M.},
  doi = {10.1007/978-0-387-87458-6}
}

HTH,

Thierry

----------------------------------------------------------------------------
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek
team Biometrie & Kwaliteitszorg
Gaverstraat 4
9500 Geraardsbergen
Belgium

Research Institute for Nature and Forest
team Biometrics & Quality Assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium

tel. + 32 54/436 185
Thierry.Onkelinx at 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 at r-project.org [mailto:r-sig-mixed-models-bounces at r-project.org] Namens Sven Wirthner
Verzonden: vrijdag 15 januari 2010 11:44
Aan: r-sig-mixed-models at r-project.org
Onderwerp: [R-sig-ME] paired and repeated measurements

Dear all

I have the following problem:

I measured co2 on 6 paired sites (one grubbed and one non-grubbed fence per site -> grubbing = treatment). These measurements I repeated
15 times over 2 years (see attached table). So, now my problem is how to analyze these data. I tried the following model:

mod1_CO2<-lme(co2~treatment+time,random=~1|site,data=CO2_t1_t15)

I think with the random effect I included the paired sample design of my study, but obviously the model does not check for the "repeated measurements" structure of my data. Or in other words, the model assumpt that different sampling dates are independent of each other, but they aren't (since I measured the CO2 15 times on exactly the same spot).

So, that's why my question is how can I combine the paired (or nested) design with the repeated measurements in one model???

Thank you for any help

Sven


--
Sven Wirthner
Eidg. Forschungsanstalt für Wald, Schnee und Landschaft WSL Tierökologie Zürcherstrasse 111
CH-8903 Birmensdorf

Tel. +41 44 7392 371
Fax  +41 44 7392 215
sven.wirthner at wsl.ch
http://www.wsl.ch

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