Vector of factor variables that will be
the only ones shown
only
logical; If factors has a value this
determines how interactions are treated. True means just
that variable will be shown and not its interactions.
False means interactions will be included.
shorten
logical or character; If FALSE then
coefficients for factor levels will include their
variable name. If TRUE coefficients for factor
levels will be stripped of their variable names. If a
character vector of variable