Missing data for a particular question or item is called item nonresponse. It occurs when a respondent fails to provide an answer to a required item. The ACS also considers invalid answers as item nonresponse. The Census Bureau uses imputation methods that either use rules to determine acceptable answers or use answers from similar housing units or people who provided the item information. One type of imputation, allocation, involves using statistical procedures, such as within-household or nearest neighbor matrices populated by donors, to impute for missing values.

• Contact List: To obtain additional information on these and other American Community Survey (ACS) subjects, see the list of Census 2000/2010 Contacts on the Internet at www.census.gov/contacts/www/c-census2000.html.
• Scope: These definitions apply to the data collected in both the United States and Puerto Rico. The text specifically notes any differences. References about comparability to the previous ACS years refer only to the ACS in the United States. Beginning in 2006, the population in group quarters is included in the data tabulations.
• Historical Census Comparability: For additional information about the data in previous decennial censuses, see www.census.gov/prod/cen2000/doc/sf4.pdf

Appendix B and subject definitions for American Community Survey years prior to 2005.

• Weighting Methodology: The weighting methodology in the 2006 ACS was modified in order to ensure consistent estimates of occupied housing units, households, and householders. For more information on the 2006 weighting methodology changes, see User Notes on the ACS website (www.census.gov/acs). There were no significant changes to the 2007 or 2008 weighting methodology. Beginning in 2009, the weighting methodology has changed to include the use of controls for total population for incorporated places and minor civil divisions.
• For subject definitions from previous years, visit www.census.gov/acs.