that housing permit data do not necessarily correspond with actual housing units constructed and occupied and given the problems of estimating demolitions and conversions to nonresidential use. The errors in the housing unit controls may well vary across geographical areas and may also cumulate over time.

While one cannot be sure what the magnitude or direction of the errors will be in the 2000 census-based estimates of the population and housing for 2010, one can be reasonably sure that there will be discrepancies between the estimates and the 2010 census counts and, furthermore, that the discrepancies will be greater for many counties and combinations of counties that are the basis for the ACS weighting controls. For the ACS, this means that there will be a discontinuity in many areas in totals for important demographic groups between 1-year period estimates for years preceding a census and for years including and following a census. This discontinuity will also exist for 3-year and 5-year period estimates between those that completely antedate a census year and those that include and follow a census year (for example, when comparing 5-year period estimates for 2005–2009 and 2010–2014). For 3-year and 5-year period estimates that span a census year (for example, a 5-year period estimate for 2008–2012), the Census Bureau plans to use an average of controls in which the population estimates for precensus years are adjusted to be consistent with the census counts.

One might consider that ACS estimates of percentages, as opposed to levels, would not be affected by the problem of differences in precensus and postcensus population controls. This will be the case, however, only if the discrepancies between the two sets of controls are relatively uniform by demographic category. If the discrepancies differ by category, which is likely, then the percentages will be affected as well (see Table 3-7 for an example).

There is no universal solution for the problem that will result from discrepancies between precensus and postcensus population controls. Users must address the situation for their applications and areas of interest, given that the problem will be more significant for some areas and population groups than others. The Census Bureau can help users in this regard by producing concurrent series of estimates that are based on precensus and postcensus controls. For example, the Census Bureau could produce two series of 1-year period estimates for, say, 2008–2010, in which the first series would use the 2000 census-based controls (the official series for those years), while the second series would backcast the 2010 census-based controls.

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