How to obtain random effects model matrix?-CodePudding

I have a model such as:

mymod = lmer(y ~ x1   x2   (x1 | id) , data = mydata)

I know I can get the model matrix from the fitted object using getME but is there a way to obtain the model matrix for the fixed effects without first fitting the model:

CodePudding user response：

You can do this using the lformula function in the lme4 package. This rerurns an object which has holds the transpose of this matrix, Zt:

library(lme4)

# create some toy data
dt <- expand.grid(x1 = 1:4, x2 = 5:6, id = LETTERS[1:20], reps = 1:2)

# this is the model in the OP:
myFormula = "y ~ x1   x2   (x1 | id)"

# for lFormula to work we need y in the data frame
# so just put a vector of 1s since that will not affect the random effects model matrix:
dt$y <- 1

Then:

foo <- lFormula(eval(myFormula), dt)
Z <- t(as.matrix(foo$reTrms$Zt))

where Z is the model matrix for the random effects that you requested.

CodePudding user response：

Try getME(lmer(y ~ x1 x2 (x1 | id) , data = mydata)).