​The U.S. Census Bureau conducts the American Community Survey (ACS) continuously, even during decennial census years. All addresses in sample for the ACS in 2020 were asked to complete both the ACS and the census, which can be confusing and burdensome to respondents. Since most households sampled for the ACS were initially contacted by mail, the mail materials were revised to help legitimize the ACS and distinguish it from the 2020 Census. This report provides results of a test assessing whether the revised mail materials achieved the goal of encouraging ACS response. It documents differences in ACS response between treatments using the March 2020 ACS panel, as well as differences in the presence and timing of a 2020 Census response.