​The 2019 ACS Due Dates Test tested the effect of the inclusion of a due date in the ACS mail materials. To encourage self-response, the U.S. Census Bureau sends up to five mailings to sampled households. The due date was placed in the fifth mailing—the last mailing that households that had not responded yet receive before the CAPI operation began. The fifth mailing was a pressure seal letter. Two variations on the placement of the due date were tested (inside the letter and on the outside of envelope), along with three different messages on the consequences of not responding by the due date.