Data Dictionary: ACS 2006 -- 2008 (3-Year Estimates)
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Data Source: U.S. Census Bureau
Table: B99104. Imputation Of Length Of Time Grandparent Responsible For Own Grandchildren Under 18 Years For The Population 30 Years And Over [7]
Universe: Universe: Population 30 years and over
Table Details
B99104. Imputation Of Length Of Time Grandparent Responsible For Own Grandchildren Under 18 Years For The Population 30 Years And Over
Universe: Universe: Population 30 years and over
Relevant Documentation:
Excerpt from: Social Explorer; U.S. Census Bureau; American Community Survey 2006-2008 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 2006-2008 Summary File: Technical Documentation.
 
Duration of Responsibility
The answer refers to the grandchild for whom the grandparent has been responsible for the longest period of time. Duration categories ranged from less than 6 months to 5 or more years.