find_start_date defaults

[nsbatra]

find_start_date and find_end_date do not appear to return the recall_start or recall_end if the other searched columns are NA. The solution was to include recall_start and recall_end in the list of searched columns, but this is not ideal. The function should return the recall start/end date if none of the other dates are found.

See the sitrep survey walkthrough for example.