sum_activity()
counts the number of active time periods (i.e., days, weeks,
months, years, or waves) by panelist_id
. A period counts as "active" if
the panelist provided at least one visit for that period.
sum_activity()
counts the number of active time periods (i.e., days, weeks,
months, years, or waves) by panelist_id
. A period counts as "active" if
the panelist provided at least one visit for that period.