[R] Specify Z matrix with lmer function
Mark Lyman
mlyman at byu.edu
Thu Nov 3 19:05:03 CET 2005
Is there a way to specify a Z matrix using the lmer function, where the
model is written as y = X*Beta + Z*u + e?
I am trying to reproduce smoothing methods illustrated in the paper
"Smoothing with Mixed Model Software" my Long Ngo and M.P. Wand.
published in the /Journal of Statistical Software/ in 2004 using the
lme4 and Matrix packages. The code and data sets used can be found at
http://www.jstatsoft.org/v09/i01/.
There original code did not work for me without slight modifications
here is the code that I used with my modifications noted.
x <- fossil$age
y <- 100000*fossil$strontium.ratio
knots <- seq(94,121,length=25)
n <- length(x)
X <- cbind(rep(1,n),x)
Z <- outer(x,knots,"-")
Z <- Z*(Z>0)
# I had to create the groupedData object with one group to fit the model
I wanted
grp <- rep(1,n)
grp.dat<-groupedData(y~Z|grp)
fit <- lme(y~-1+X,random=pdIdent(~-1+Z),data=grp.dat)
I would like to know how I could fit this same model using the lmer
function. Specifically can I specify a Z matrix in the same way as I do
above in lme?
Thanks,
Mark
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