How to get coefficients and their confidence intervals in mixed effects models?

Question

In lm and glm models, I use functions coef and confint to achieve the goal:

m = lm(resp ~ 0 + var1 + var1         


        

Answer 1

Assuming a normal approximation for the fixed effects (which confint would also have done), we can obtain 95% confidence intervals by

estimate + 1.96*standard error.

The following does not apply to the variance components/random effects.

library("lme4")
mylm <- lmer(Reaction ~ Days + (Days|Subject),  data =sleepstudy)

# standard error of coefficient

days_se <- sqrt(diag(vcov(mylm)))[2]

# estimated coefficient

days_coef <- fixef(mylm)[2]

upperCI <-  days_coef + 1.96*days_se
lowerCI <-  days_coef  - 1.96*days_se