Data Dictionary: ACS 2008 (1-Year Estimates)
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Data Source: U.S. Census Bureau
Universe: Universe: Civilian noninstitutionalized population 5 years and over
Relevant Documentation:
Excerpt from: Social Explorer; U.S. Census Bureau; American Community Survey 2008 Summary File: Technical Documentation.
 
Imputation Rates
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.
Overall Person Characteristic Imputation Rate
This rate is calculated by adding together the weighted number of allocated items across a set of person characteristics, and dividing by the total weighted number of responses across the same set of characteristics.
Overall Housing Characteristic Imputation Rate
This rate is calculated by adding together the weighted number of allocated items across a set of household and housing unit characteristics, and dividing by the total weighted number of responses across the same set of characteristics. These rates give an overall picture of the rate of item nonresponse for a geographic area.

Excerpt from: Social Explorer; U.S. Census Bureau; American Community Survey 2008 Summary File: Technical Documentation.
 
Ambulatory Limitations
The data on ambulatory functioning were derived from answers to Question 17b, which was asked of people 5 years old and over. The question asked respondents if they had "serious difficulty walking or climbing stairs." Respondents were instructed to mark "yes" or "no." Question 17b is labeled as "Ambulatory difficulty" for some disability data products such as the ACS Detailed Tables.
Limitation of the Data
The Census Bureau does not recommend trend analysis using the 2008 data with years prior to 2008 due to the questionnaire change.
Question/Concept History
The 2008 American Community Survey marks a break in the series of disability data. In prior American Community Surveys, a similar question about "conditions that limit one or more basic physical activities such as walking, climbing stairs, reaching, lifting, or carrying" was asked However, the changes to the wording and tailoring of the list of activities on which the limitation is based suggest that the 2008 measure is not comparable with the "Physical disability" estimates from prior years. For additional information on the differences between the 2008 ACS disability questions and prior ACS disability questions, see "2006 ACS Content Test Evaluation Report Covering Disability" (http://www.census.gov/acs/www/AdvMeth/content_test/P4_Disability.pdf).