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Using the American Community Survey: Benefits and Challenges
there will likely be characteristics for which the resulting estimates for some areas will be seriously biased due to the pattern of changes in the characteristic over the period. The use of a period estimand, averaged over the time period, avoids this concern about bias. The Census Bureau has decided to adopt the approach of period estimation for the multiyear data from the ACS, which, under the assumptions made above in Section A.1, also leads to the equal weighting scheme and hence lowest variance. However, the period estimation approach achieves its benefits by placing considerable burden on users to interpret the estimates in an appropriate manner.
As discussed in Section 3-C.1.b and later in this chapter, period estimates are difficult to interpret, and users must assess them in terms of external information about changes that have occurred during the period. For example, a 5-year period estimate of the percentage of poor families of 10 percent could reflect any of the following: a constant percentage across the 5 years; a steady increase from, say, 7 percent to 13 percent; a corresponding steady decrease; a rise and decline in the percentage across the years; and so on. To obtain an indication of the likely pattern that underlies a 5-year (or 3-year) estimate, users need to apply local knowledge of the conditions in the area over the period. They can also examine the published 1-year estimates for a larger area that contains the area of interest.
6-B MULTIYEAR PERIOD ESTIMATION
Conceptually, multiyear period estimation is the same as 1-year period estimation, merely extended over a longer period. The starting point for producing multiyear period estimates is to concatenate the 1-year data files over the 3 or 5 years involved, in the same way that 1-year estimation is based on concatenating the monthly data collected within a calendar year. Then the weighting scheme the Census Bureau is proposing for multiyear estimation from the concatenated file is broadly the same as that used for the 1-year estimation, as described in Chapter 5.
A natural and very simple way to develop weights for use with a multiyear concatenated file is to take the existing weights on each of the 1-year files and divide them by the number of years involved (3 or 5). A variant of this simple approach takes advantage of revised, updated housing and population controls for earlier years in the period, which may have become available by the time when the weights for the period estimates are being developed. Under this variant, the 1-year weights would be revised by using the applicable updated housing and population controls, and the revised weights would then be divided by 3 or 5.
The Census Bureau is planning, however, to use a similar but somewhat different approach. In its method, the first two steps in the 1-year weighting process (that is, base weights and variation in monthly response factor—see