# Formula for lmer() [duplicate]

How does the formula for the lmer function work?

Some examples:

library('lmer')
lmer(Yield~(1|Batch),Dyestuff)
lmer(Yield~(Batch)*(1|rep(c(1,2,3),10)),Dyestuff)
lmer(Reaction ~ Days + (1|Subject) + (0+Days|Subject), sleepstudy)


On the left of ~ sign is the dependent variable and on the right comes all the independent variables separated by the + sign.

Is there any difference between + and *?

What does 1|Batch mean? Does it have to do with the intercept? With being a random or a fixed effect?

What does Day|Subjects mean?

## marked as duplicate by chlMay 15 '13 at 10:18

• (1|Batch) means "one intercept per batch". – Stéphane Laurent May 15 '13 at 8:36
• (Days|Subject) means a random intercept and a random slope for each subject. (1|Subject) would mean only a random intercept but a common slope across subjects. – COOLSerdash May 15 '13 at 9:18