R Under development (unstable) (2023-07-03 r84633 ucrt) -- "Unsuffered Consequences"
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> require(lme4)
Loading required package: lme4
Loading required package: Matrix
> # sorry for fitting yet another sleepstudy model in the tests
> m <- lmer(Reaction ~ Days + (Days | Subject), sleepstudy)
> ST <- getME(m, "ST")$Subject
> 
> # copied from vince dorie's simmer.R in arm:
> dimension <- nrow(ST)
> T <- ST
> diag(T) <- rep(1, dimension)
> S <- diag(diag(ST), dimension)
> 
> vc0 <- getME(m, 'sigma')^2*tcrossprod(T %*% S)
> vc1 <- VarCorr(m)$Subject[,]
> dimnames(vc0) <- dimnames(vc1)
> 
> all.equal(vc0, vc1, tolerance = 1e-6)
[1] TRUE
> 
> proc.time()
   user  system elapsed 
   1.09    0.17    1.26