In 2005, the American Community Survey (ACS) began full - scale data collection in all counties in the United States. The ACS is an annual survey that collects data from housing units and group quarters, but this evaluation focuses on housing units only. The ACS collects data from housing units using three response modes: mail, Computer - Assisted Telephone Interviewing (CATI), and Computer - Assisted Personal Interviewing (CAPI). Selected mailable addresses are sent a mail questionnaire package at the beginning of the month they are selected to be in sample. If the Census Bureau has not received the questionnaire after three weeks, a second mail questionnaire is sent. Those who do not return the questionnaire by the end of the first month and have a telephone number available are contacted for a CATI interview in the second month. Finally, a subsample of those who do not respond by either mail or telephone in the first or second month and a subsample of unmailable addresses are contacted for a CAP I interview in the third month. This evaluation aimed to identify demographic and socio - economic characteristics that are associated with households responding by mail, CATI, and CAPI.