The sort of standard error sought. If clusters is
not specified the options are "HC0", "HC1" (or "stata", the equivalent),
"HC2" (default), "HC3", or "classical". If clusters is specified the
options are "CR0", "CR2" (default), or "stata". Can also specify "none",
which may speed up estimation of the coefficients.
clusters
A vector corresponding to the clusters in the data.
ci
logical. Whether to compute and return p-values and confidence
intervals, TRUE by default.