terns in the underlying 1-year estimates. For example, a 2 percent change between two nonoverlapping 3-year estimates could occur because all the increase—a 6 percent increase—occurred in the latest year, because a 2 percent increase occurred in the interval between the two estimates, or because of other patterns of change. Users need to be aware of the possible underlying patterns and find ways to distinguish between them based on other sources or on ACS data at other levels of aggregation.

6-D
EFFECTS OF CHANGES IN POPULATION SIZE AND CHARACTERISTICS

The treatment thus far in this chapter has assumed that the population of an area has been static, or at least has changed in only minor ways, during the time period of a 3- or 5-year period estimate. While this may be a reasonable assumption for many areas, there will be some areas that experience major changes in population size or composition or both. Moreover, major changes are most likely to occur in small governmental units and census tracts, areas for which 3-year and 5-year estimates are needed.

Population changes that recur within a year, such as the seasonal patterns discussed in Section 3-C.3, affect each year of a multiyear period estimate in the same way that they affect a 1-year period estimate. The additional population changes that affect multiyear period estimates are year-to-year changes, such as population growth over time, which may be concentrated in certain demographic subgroups. The Census Bureau’s planned weighting procedures for multiyear estimates reflect such changes by using the 3- or 5-year averages of the independent housing unit estimates and the independent population estimates by demographic subgroup as controls.

Users need to consider the potential effects of the planned weighting scheme on ACS estimates. For this discussion, it is useful to distinguish between two types of ACS estimates: estimates of proportions, such as the proportion of poor people, and estimates of totals, such as the number of poor people. In the case of proportions, if an area had major growth, say, an influx of young persons, the ACS multiyear period estimates of the characteristics of young people will be weighted toward their characteristics in the later years of the period, and the influx will similarly affect other estimates in which young people are included. Even if it seems likely that a characteristic of interest has remained stable over the period, the changes in the population composition will lead to differences in the proportions of the population with that characteristic over the time period. Consider, for instance, the comparison of the multiyear estimates of the unemployment rates between this area and a stable or declining area. In this case, users



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