Unit nonresponse is the failure to obtain the minimum required information from an eligible housing unit in the sample. Unit nonresponse occurs when respondents are unable or unwilling to participate, interviewers are unable to locate addresses or respondents, or when other barriers exist to completing the interview.

It is important to measure nonresponse because it can have a direct effect on the quality of the data. If the rate of unit nonresponse is high, it increases the chance that the final survey estimates may reflect bias. Estimates may reflect bias if the characteristics of nonresponding units differ from the characteristics of responding units.

Housing unit response rates, which measure the unit nonresponse, are one of several measures commonly used as indicators of a survey's quality. Response and nonresponse rates are complementary (nonresponse rate = 100 percent - response rate), and can be used interchangeably when discussing a survey’s quality. The American Community Survey (ACS) generally focuses on response rates, but both rates are presented in this report.