reference period for the 2000 long-form sample was a uniform January–December 1999. It is possible that the C2SS captured the onset of recession in a way that the 2000 long-form sample could not. The inflation adjustment procedure for the C2SS could also be a factor. For the Census Bureau’s comparative analysis, the C2SS data were backward adjusted to reflect the average inflation experience for 1999, not 2000. Analysis by Turek, Denmead, and James (2005) suggests that inflation adjustments to the ACS may not accurately reflect economic growth (or decline) over a year.

2-C
ACS CHALLENGES

The ACS will benefit users by providing more timely and frequent data for small areas that are likely to be based on responses of higher quality than the census long-form sample. However, there are challenges to using the data that stem principally from the continuous design of the ACS. Two major challenges are (1) the period nature of the ACS 1-year, 3-year, and 5-year estimates in contrast to the point-in-time nature of the long-form-sample estimates (Section C.1) and (2) the greater sampling error of the ACS estimates compared with the long-form-sample estimates (Section C.2).

2-C.1
Period Estimates

All estimates from the ACS will be period estimates—that is, the estimates will represent averages of months of data—12 months for 1-year estimates, 36 months for 3-year estimates, and 60 months for 5-year estimates. The issue is how to interpret the period estimates from the ACS, which are not the same as the (approximately) point-in-time estimates for Census Day (April 1) from the census long-form sample.

Consider first the 1-year period estimates, which include responses in all 12 months of the calendar year for sampled housing units that existed on the MAF as of January of the year (see Section 4-A). An independent estimate of total housing for July 1 is used to control the estimated number of housing units in an estimation area, but the reported characteristics of the units may vary throughout the year. Consequently, for housing characteristics (utility costs, value, rent, number of rooms, and others), the 1-year period estimates are 12-month averages, which may often differ from long-form-type point-in-time estimates.

Similarly, even though independently derived census-based population estimates for July 1 for major age, sex, race, and ethnicity groups are used to control the 1-year period estimates of people, such characteristics as education, income, and others may vary during the year. The 1-year period estimates are consequently 12-month averages of such population characteristics as education, income, veterans status, and others. A 1-year period estimate for an area will correspond to a point-in-time estimate for July



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