How do I extract the Correlation of fixed effects part of the lmer output

Question

When you have a multilevel model with lots of factors and interactions the size of the correlation of fixed effects matrix can become quite big and unclear.

I can us

Answer 1

Using the S4 method listing function we can return the function which is dispatched when print is called on an object of class "mer":

selectMethod( print , "mer" )

Looking at the source code that is returned we can find the lines applicable to you:

if (correlation) {
            corF <- so@vcov@factors$correlation

so is defined as the summary of your object, so in your case you should just need to extract:

so <- summary(mod)
so@vcov@factors$correlation

Answer 2

How about using the built-in

cov2cor(vcov(mod))

?

Answer 3

I don't know direct method. But this is workaround.

 diag(diag(1/sqrt(vcov(mod)))) %*% vcov(mod) %*% diag(diag(1/sqrt(vcov(mod))))