and the 2004 ACS supplementary survey produced about the same level of total income (Nelson, 2006). There were differences by source (the CPS estimated more wages and less self-employment income compared with the ACS), but the aggregates were very close.

Research is needed to understand the contributions of each of the above factors to differences between the CPS ASEC and the ACS. For users, now that the ACS is in full production with a vastly larger sample size than the CPS ASEC, it seems reasonable that they look to the ACS estimates for states and substate areas. However, users who want to analyze income by source and examine the correlates of income for population groups at the national level should stay with the CPS ASEC, which not only is the source of official income statistics, but also contains a wealth of variables to use in analysis.

3-G
WHAT HAPPENS IN A DECENNIAL YEAR?

An important element of the ACS design is to control each year’s estimates at the level of the county (or group of small counties) by total housing and by total population, categorized by age, sex, race, and Hispanic origin. The population control totals for each year are produced by updating the previous decennial census totals with administrative records on births, deaths, and net migration. The housing unit control totals for each year are produced by updating the previous census totals with housing permit records (see Chapter 5). The use of control totals is important to reduce sampling error in the estimates and to adjust the ACS estimates for possible undercoverage of housing and of the population, which may be particularly pronounced for some demographic groups.

The problem with the population control totals is that they become increasingly prone to error as each year passes from the previous census. While birth and death records are very accurate, there is considerable uncertainty about the quality of estimates of net migration, both net immigration from abroad, including illegal immigration, and net migration flows among counties. In the 2000 census, the estimate of the total U.S. population updated from the 1990 census was 1.8 million people fewer than the 2000 census count of 281.4 million people, and there were significant errors also in estimating the population of subnational areas. The national underestimate, which was particularly large for people ages 18–29 and for minorities, was attributed to an underestimation of illegal immigrants during the economic boom of the last half of the 1990s (National Research Council, 2004b:Table 5.1).

The postcensal housing unit controls are also subject to error, given



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