Predicting for new factor levels in a mixed effects model - lme4

lme4-predict-new-ranef-levels.R

library(lme4)

# adapted from ?predict.merMod
gm1 <- glmer(cbind(incidence, size - incidence) ~ period + (1 |herd), 
             cbpp, 
             binomial)

# add some new levels to herd, which is a random effect
newdata <- with(cbpp, 
                expand.grid(period=unique(period), 
                            herd=c(unique(herd), 16:20)))

# this raises an error
predict(gm1,newdata)

# this allows you to make predictions for new ranef levels
predict(gm1, newdata, allow.new.levels = TRUE)