syntax lmer random effects

Thu, Jul 26, 2007 10:52 AM

Nathan:

Here are a few small examples given what you have provided below. To use lmer with an intercept only and random intercepts:

fit1 <-  lmer(z ~ 1 + (1|ID), data, method='ML')

Now, to fit a model with a fixed effect, 'x', and to allow for the intercept and for the variable x to vary randomly, use the following:

fit2 <-  lmer(z ~ x + (x|ID), data, method='ML')

Notice that the random terms are always (x|ID), where x is the variable you want to have as the random effect, followed by a "pipe" (|) and then the grouping variable. In this example the grouping variable is ID. So, you have a random variable x, "given", or "conditional on" ID.

Also notice I use method='ML'. You said you wanted to compare models and these models differ in their fixed effects. REML is the default estimation procedure, and so you need this portion of code.

syntax lmer random effects

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