if not NULL, a vector whose entries are the variable
names in the dataset corresponding to each summation
question. if NULL, then assume that
the survey.data dataframe has an attribute called 'sum.qs'
containing this vector.
missing
if "ignore", then proceed with the analysis without
doing anything about missing values. other options
are not yet implemented.
Value
a vector with an estimate of the degree for each row
in survey.data. if na.rm=TRUE, then the degree for rows that have
missingness in the summation questions will be set
to NA
Details
TODO -- cite summation method ref
Note that the summation degree estimator for the
case where there is missing data is not yet implemented.
(In fact, I don't think that there is a known
estimator for this case.)