A vector of character strings containing the grouping levels, from most general to most specific
covariates
A list with names corresponding to effects and each element being a vector of covariates applicable at that level
observations
A character string giving the column of observations, or a vector where the first element is the observations and the remaning are offsets. For binomial responses, the first element is the counts (of successes), and the second element is the total number
data
A data frame containing the response, covariates, and group membership.