estimates for small governmental jurisdictions. A number of steps were taken to achieve high coverage for the DMAF and to remove duplicate and nonexistent addresses. Although the final DMAF had some remaining errors of omission and erroneous inclusion (as noted above), there is evidence that, on balance, it included virtually all of the housing stock at the time of the census in spring 2000 (National Research Council, 2004b:140-141). In turn, the long-form sample represented the characteristics of the housing stock as of spring 2000 and the people living in that housing as of springsummer 2000 (allowing that not all enumerations were completed until August and that there were undoubtedly errors in reporting where people lived as of Census Day, April 1). Recall that long-form-sample questions refer to the time of the census or the prior calendar year and that long-form-sample population and housing estimates are adjusted to agree with Census Day control totals from the complete count census of both short and long forms for small weighting areas.

The ACS representation of housing and people in any one year differs in at least three respects from the point-in-time long-form-sample representation for the census year. These differences involve: (1) the composition of the housing sample each year; (2) the composition of the sample of people in the sampled housing stock, and (3) the housing unit and population estimates that are used as ACS controls.

  1. The ACS sample of housing units in a given year t represents addresses recorded on the MAF as of January of year t; there is no provision to add newly constructed or newly identified units until the following year.5 Given that the ACS collects data continuously throughout the year, measures of the characteristics of the January housing stock from the ACS (for example, occupancy status, owner/renter, housing value) represent averages over the entire year, which may not be the same as how these characteristics would be measured in January. For housing units newly added to the MAF between August of year t – 1 and January of year t, they are included in the year t sample for only 9 months of the year (April-December), so their characteristics (for example, vacancy status) will represent averages over the last 9 months rather than all 12 months of the year.

  2. The ACS sample of household members in year t represents people who lived in the January MAF housing stock at some time in the

5

Each year’s ACS estimates actually pertain to responses obtained during that year, some of which may come from housing units that were included in the sample for November or December of the preceding year but did not provide data until January or February. This does not materially affect the discussion in the text.



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