[R-sig-ME] specification of conditional random effects in lme
albrechj at staff.uni-marburg.de
albrechj at staff.uni-marburg.de
Thu Oct 11 17:09:41 CEST 2012
Hello,
I have a question regarding the correct specification of the
conditional random effects in the function lme in package nlme.
Consider the following dataset:
str(data1)
'data.frame': 18 obs. of 6 variables:
$ site : Factor w/ 10 levels "a","b","c",...
$ year : Factor w/ 2 levels "2011","2012"
$ covariate : num
$ FactorA : Factor w/ 2 levels "0","1"
$ FactorB : Factor w/ 2 levels "0","1"
$ Y : num
I want to analyse the effect of the covariate and the main factors A
and B and their interaction A x B on the response Y. The data has been
collected in 10 study sites in 2011 and in 8 of these sites in 2012.
In the specification of the linear mixed effects model I would like to
fit year as a conditional random effect on site.
So my question is, whether I have to include year in the specification
of the "fixed effect" as well:
fit1 <- lme(fixed = Y ~ covariate + (A + B)^2 + year, random = ~
year|site, data = data1)
or whether I can omit year from the fixed effects, when I am not
interested in the effect of year, but just want to account for a
potential effect of year:
fit2 <- lme(fixed = Y ~ covariate + (A + B)^2, random = ~ year|site,
data = data1)
Is there anything fundamentally wrong in the second way of model
specification?
Sincerely,
Jörg
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