Optionally, the survey can have an attribute called label, which is the
long name of the survey. Optionally, each variable in the survey can have an
attribute called label, which is the variable's long name.
For age-adjustment, aa_pop$Population can contain either population counts
or proportions/weights for each level. Values are normalized internally, so
counts and proportions produce the same age-adjusted estimates when they
describe the same standard population distribution.