year according to the residence rules for the ACS, which uses a 2-month rule and not the census “usual residence” rule (see Section B.2 below). The measures of household member characteristics in year t from the ACS represent averages over the entire year. Reflecting the continuous data collection in the ACS, the questionnaire items (population and housing) refer to the time during the year when respondents fill out the survey form, or, for some items, to the previous 12 months.
The housing units and people for whom the ACS collects data in year t are weighted to agree with independently derived estimates of total housing units and population by age, race, sex, and ethnicity as of July 1 for each of 1,951 estimation areas (described above). The application of July 1 controls may make it appear that the ACS representation of housing and population is not so different from the long-form sample, which represents April, but this is not the case. As explained below, the July 1 controls, which derive from the previous census, are not consistent with the underlying ACS data.
For 1-year period estimates from the ACS for housing, the application of a single control for total housing units—assuming it is accurate—will capture growth (or decline) in the housing stock in an estimation area that occurred between January and June. However, it will not capture changes in the composition of the housing stock—for example, in single-unit versus multiunit dwellings—due to growth (or decline) between January and June, nor will it capture changes in the housing stock between July and December. Moreover, the application of housing unit controls for estimation areas will not capture differences in housing growth (or decline) among smaller areas within an estimation area, such as cities for which independent housing unit estimates are available but are not currently used as ACS controls. Finally, the ACS estimates of characteristics of the January housing stock will be averages over the year and not point-in-time estimates for July 1 or any other time during the year. (See Section 5-C for further discussion of the housing unit controls.)
For 1-year period estimates from the ACS for people living in the January housing stock, the application of the population controls will adjust a few more dimensions than just total population to a July 1 reference date—namely, sex, age (13 categories), and race and ethnicity (6 categories), although in practice some collapsing of the cross-classification of these dimensions is common. Yet the population adjustments will have all of the problems of the housing unit adjustments enumerated above. In addition,