weights
if not NULL, weights to use in computing the estimate. this
should be the name of the column in the survey.data which has
the variable with the appropriate weights. these weights
should be construted so that, eg, the mean of the degrees is
estimated as (1/n) * \sum_i w_i * d_i
missing
if "ignore", then proceed with the analysis without
doing anything about missing values. if "complete.obs"
then only use rows that have no missingness for the
computations (listwise deletion). care
must be taken in using this second option