Description of the data source, numerator, denominator, survey questions, and other relevant details about the national estimate.
From the American Housing Survey, 2007:
NOSTEP entrance accessible without steps or stairs
In cooperation with the U.S. Department of Housing and Urban Development, the Census Bureau conducts the American Housing Survey, a biennial survey of the nation's housing market. The AHS is conducted by the Census Bureau’s 12 regional offices using telephone and personal visit interviews. The Bureau staff make personal visits if the housing unit is new to the AHS sample, if the household living in the sampled unit has changed since the previous interview, or if the period between the interviews exceeds several years, which is the case for the metropolitan areas.
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