[R] paired repeated measurements

[Bert Gunter]

Sven:

1. Try this question on the r-sig-mixwed-models list -- more likely to get
good responses there, I think.

2. However, my less expert opinion is that you need a random fence within
site term (each fence has it's own ID)

co2 ~ treatment + time, rand = ~1|fence/site

This assumes time is continuous and not a factor, btw; if time is a factor,
I think there may be identifiability problems -- but folks on the mixed
models list should be able to provide you better guidance. There may well be
better ways to model the covariance structure.

Bert Gunter
Genentech Nonclinical Biostatistics
 
 -----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
Behalf Of Sven Wirthner
Sent: Thursday, January 14, 2010 5:24 AM
To: r-help at r-project.org
Subject: [R] paired repeated measurements

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. 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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