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AMERICAN COMMUNITY SURVEY RESEARCH NOTE

Change in Population Controls

September 22, 2011

The American Community Survey (ACS) uses independently derived population estimates as population 

controls in its weighting methodology. While the ACS uses updated population estimates every year, the 

update in the population estimates from 2009 to 2010 was particularly significant. The 2010 ACS 1-year 

estimates use population estimates that reflect the results of the 2010 Census. The 2009 ACS 1-year 

data and ACS 1-year estimates for prior years used population estimates that were based on the 2000 

Census results and updated to account for births, deaths, domestic and international migration (these 

estimates are referred to as the postcensal estimates). 

Because the 2009 ACS and 2010 ACS 1-year estimates use controls that are based on different Census 

base years, users should use caution when making comparisons across years. Specifically, estimates of 

the number of people in a given location are not strictly comparable between these two years. The 

characteristic distributional measures (such as rates or ratios) are not expected to be substantially 

affected by the change in population controls. The Census Bureau is currently researching the impact on 

these distributions.

This Research Note provides information about the population estimates used in the 2010 ACS 

weighting methodology. Further, it describes some possible inconsistencies between the 2010 ACS 1-

year estimates which used population estimates updated to include results of the 2010 Census and the 

2009 ACS 1-year estimates which used population estimates based on Census 2000.

How the ACS Uses Population Estimates

The ACS is a probability sample of housing units and group quarters designed to produce national, state 

and local estimates of the distribution of characteristics of the population. The demographic 

characteristics of the ACS sample can differ from known population levels due to sampling variability 

and differential coverage.

1 To account for differences in demographic characteristics due to differential 

coverage, the ACS uses a weighting methodology that forces ACS estimates to be consistent with official 

population estimates by age, sex, race, and Hispanic origin.

2 These specific estimates are called

population controls. Data users may be aware of these from the notes on some tables in American 

FactFinder.3

The postcensal population estimates used for the 2000 through 2009 ACS were based on Census 2000 

results updated with annual changes in births, deaths, domestic and international migration. As in past 

 1

One of the sources of undercoverage is non-response or non-participation. If a specific demographic group is less 

likely to respond, then the weighting methodology recognizes and accounts for that so that the final estimates are 

representative of the population. 

2

Chapter 11 “Weighting and Estimation” of the ACS Design and Methodology Report provides additional details on 

weighting and population estimates in the ACS. 

<http://www.census.gov/acs/www/methodology/methodology_main/>

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When an ACS estimate has this forced consistency, the Census Bureau does not produce an estimate of variance. 

In American FactFinder, these "controlled" statistics are noted as follows: “An entry of '*****' in the margin of 

error column indicates that the estimate is controlled. A statistical test for sampling variability is not appropriate. 

For those characteristics that are controlled, the change between any two years is the result of updating the 

population estimates.
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decades, the population estimates are modified to incorporate the new decennial census counts as soon 

as they are available. The 2010 ACS population controls were developed based on the 2010 Census 

results. 

Analysis of the Differences Between Population Estimates based on the 2010 Census and those Based 

on Census 2000

Table 1 presents a comparison between two sets of estimates: the postcensal estimates of the

population for July 1, 2010 (these postcensal estimates are based on Census 2000 and do not 

incorporate the 2010 Census results) and estimates for July 1, 2010 that are based on Census 2010.4

 At 

the national level, the difference between the two sets of estimates is small -- the Census 2010 based 

estimates are slightly higher than the postcensal estimate based on Census 2000. The difference is less 

than 300,000 people or 0.10 percent. This small difference occurred because the difference between 

the Census 2010 result and the postcensal estimate for April 1, 2010, is the smallest observed in recent 

decades. 

At the state level, the percent differences between the two sets of estimates for July 1, 2010 were larger 

but still generally modest. For the majority of states, the two sets of estimates are within one percent of 

each other. The percent difference for the most populous states were very small. Specifically, the 

difference for California was less than 83,000 people or 0.2 percent and for Texas it was less than 44,000

people or 0.2 percent.

When the estimates are disaggregated by age and sex, there are some differences between the two sets 

of estimates (see Table 2). At the national level, the differences between the postcensal estimates and

estimates based on the 2010 Census are largest for the youngest age group, those under age 5. For this 

age group, the estimates based on the 2010 Census are lower than the postcensal estimates by about 5 

percent for both males and females. This is a pattern seen across decennial censuses: the age group 

under 5 seems to be smaller than expected in the census when compared to postcensal estimates. Note 

that the age group under 5 in the postcensal estimates is based almost solely on data on births as both 

migration and mortality have a minor impact on this age group.

For those aged 25 to 34, the estimates based on the 2010 Census are lower than the postcensal 

estimates by 1.7 percent. This is mainly due to the difference between the two sets of estimates for 

men in this age group, where the difference is 3.3 percent compared to 0.2 percent for women. This 

age group can be heavily influenced by international migration and these differences may reflect recent 

migration out of the country that was reflected in the census but not the postcensal estimates.

There is also a difference between the two sets of estimates for the older population, those aged 75 and 

over. Again, the estimates based on the 2010 Census are lower than the postcensal estimates for this 

age group. The difference is larger for men (3.1 percent) than women (2.4 percent). 

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Note that the multiyear ACS data are controlled to several years of population estimates. The estimates used for 

the multiyear ACS are referred to as intercensal estimates and take into account both Census 2000 and the 2010 

Census data. 
3

Finally, there were differences in the controls by race and Hispanic origin (see Table 3).

5

 Table 3 

presents the set of estimates for the racial and ethnic groups that are used as controls for the ACS. 

These groups include 5 non-Hispanic race alone categories (White, Black, American Indian and Alaska 

Native, Asian, and Native Hawaiian or Pacific Islander) and the Hispanic population of any race. The data 

are also disaggregated by sex.

The smallest percent differences are seen for the White and Black populations. The estimates based on 

the 2010 Census are just over 1 percent lower for Whites than the postcensal estimates. The difference 

for Blacks is much smaller, 0.1 percent lower in the estimates based on the 2010 census. The estimates 

based on the 2010 Census for American Indians and Alaska Natives are lower than the postcensal 

estimate by about 5 percent. 

For Asians, Native Hawaiians and Pacific Islanders, and Hispanics, the estimates based on the 2010 

Census are higher than the postcensal estimates. At the national level, the difference for Asians was 5.3 

percent, the difference for Native Hawaiians or Pacific Islanders was 9.4 percent and the difference for 

Hispanics was 2.6 percent. Note there was a larger difference for Hispanic women (4.7 percent) than for 

Hispanic men (0.5) percent. These differences (smaller than the differences in the Census 2000 

evaluation of population estimates) reflect improvements in the postcensal estimates, particularly the

improvements in the estimation of net international migration, which is significant for this demographic 

group. 

Summary

Since the ACS weighting process controls to population totals by age, sex, race, and Hispanic origin, the 

impact of differences in the 2009 and 2010 ACS population controls will be observed in estimates of the 

number of people in a particular location. The change in controls is not expected to have a meaningful

impact in the percent distributions, rates, or ratios for non-demographic characteristics for many of the 

largest geographic areas. The Census Bureau is currently researching the impact of the change of 

controls on these characteristics.

The best way to measure the effect of the change in population controls on percent distributions, rates, 

or ratios of specific distributions would be is to develop a set of ACS weights using the 2010 postcensal 

population estimates based on Census 2000 – those that do not incorporate the 2010 Census results. 

The Census Bureau plans to use alternative weights to estimate the subject-specific distributions and 

examine the differences. Results from this examination will be available in Fall 2011.

In the absence of such analysis, the Census Bureau does not have the data to provide more specific 

guidance. Users are urged to use caution in ACS comparisons between years especially for distributions 

that are highly correlated to demographic characteristics used in the ACS weighting process.

 5 Note that the population estimates used as controls do not include the “Some Other Race” category. The original 

race data from Census 2000 and the 2010 Census are modified to reclassify the "Some Other Race" category. 
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Table 1. Comparison of the National and State Total Resident Population: July 1, 2010

Geography

Vintage 2010 

Postcensals Estimates 

Census 2010 Based 

Estimates

Differences

Numeric Percent

United States 309,050,816 309,349,689 298,873 0.1

 Alabama 4,729,656 4,785,298 55,642 1.2

 Alaska 708,862 713,985 5,123 0.7

 Arizona 6,676,627 6,413,737 -262,890 -3.9

 Arkansas 2,910,236 2,921,606 11,370 0.4

 California 37,266,600 37,349,363 82,763 0.2

 Colorado 5,095,309 5,049,071 -46,238 -0.9

 Connecticut 3,526,937 3,577,073 50,136 1.4

 Delaware 891,464 899,769 8,305 0.9

 District of Columbia 610,589 604,453 -6,136 -1.0

 Florida 18,678,049 18,843,326 165,277 0.9

 Georgia 9,908,357 9,712,587 -195,770 -2.0

 Hawaii 1,300,086 1,363,621 63,535 4.9

 Idaho 1,559,796 1,571,450 11,654 0.7

 Illinois 12,944,410 12,843,166 -101,244 -0.8

 Indiana 6,445,295 6,490,621 45,326 0.7

 Iowa 3,023,081 3,049,883 26,802 0.9

 Kansas 2,841,121 2,859,169 18,048 0.6

 Kentucky 4,339,435 4,346,266 6,831 0.2

 Louisiana 4,529,426 4,544,228 14,802 0.3

 Maine 1,312,939 1,327,567 14,628 1.1

 Maryland 5,737,274 5,785,982 48,708 0.8

 Massachusetts 6,631,280 6,557,254 -74,026 -1.1

 Michigan 9,931,235 9,877,574 -53,661 -0.5

 Minnesota 5,290,447 5,310,584 20,137 0.4

 Mississippi 2,960,467 2,970,036 9,569 0.3

 Missouri 6,011,741 5,996,231 -15,510 -0.3

 Montana 980,152 990,898 10,746 1.1

 Nebraska 1,811,072 1,830,429 19,357 1.1

 Nevada 2,654,751 2,704,642 49,891 1.9

 New Hampshire 1,323,531 1,316,759 -6,772 -0.5

 New Jersey 8,732,811 8,801,624 68,813 0.8

 New Mexico 2,033,875 2,065,932 32,057 1.6

 New York 19,577,730 19,392,283 -185,447 -0.9

 North Carolina 9,458,888 9,561,558 102,670 1.1

 North Dakota 653,778 674,499 20,721 3.2

 Ohio 11,532,111 11,536,182 4,071 0.0

 Oklahoma 3,724,447 3,761,702 37,255 1.0

 Oregon 3,855,536 3,838,957 -16,579 -0.4

 Pennsylvania 12,632,780 12,709,630 76,850 0.6

 Rhode Island 1,056,870 1,052,886 -3,984 -0.4

 South Carolina 4,596,958 4,636,312 39,354 0.9

 South Dakota 820,077 816,463 -3,614 -0.4

 Tennessee 6,338,112 6,356,897 18,785 0.3

 Texas 25,213,445 25,257,114 43,669 0.2

 Utah 2,830,753 2,776,469 -54,284 -1.9

 Vermont 622,433 625,960 3,527 0.6

 Virginia 7,952,119 8,024,617 72,498 0.9

 Washington 6,746,199 6,744,496 -1,703 0.0

 West Virginia 1,825,513 1,853,973 28,460 1.6

 Wisconsin 5,668,519 5,691,047 22,528 0.4

 Wyoming 547,637 564,460 16,823 3.1

Source: US Census Bureau Population Estimates Program 
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Table 2. Comparison of the National Resident Population by Sex and Age: July 1, 2010

Vintage 2010 

Postcensals 

Estimates 

Census 2010 Based 

Estimates

Differences

Numeric Percent

Both Sexes 309,050,816 309,349,689 298,873 0.1

 0-4 years 21,262,402 20,200,529 -1,061,873 -5.0

 5-14 years 40,735,102 41,076,420 341,318 0.8

 15-24 years 43,313,974 43,627,025 313,051 0.7

 25-34 years 41,981,483 41,247,075 -734,408 -1.7

 35-44 years 40,778,665 40,981,425 202,760 0.5

 45-54 years 44,437,638 45,013,249 575,611 1.3

 55-64 years 36,003,484 36,766,385 762,901 2.1

 65-74 years 21,432,543 21,841,372 408,829 1.9

 75 years and over 19,105,525 18,596,209 -509,316 -2.7

Males 152,712,500 152,107,993 -604,507 -0.4

 0-4 years 10,867,070 10,317,894 -549,176 -5.1

 5-14 years 20,834,822 20,994,448 159,626 0.8

 15-24 years 22,421,620 22,319,109 -102,511 -0.5

 25-34 years 21,441,451 20,739,220 -702,231 -3.3

 35-44 years 20,425,462 20,396,050 -29,412 -0.1

 45-54 years 21,864,682 22,148,815 284,133 1.3

 55-64 years 17,371,046 17,738,809 367,763 2.1

 65-74 years 9,956,332 10,160,744 204,412 2.1

 75 years and over 7,530,015 7,292,904 -237,111 -3.1

Females 156,338,316 157,241,696 903,380 0.6

 0-4 years 10,395,332 9,882,635 -512,697 -4.9

 5-14 years 19,900,280 20,081,972 181,692 0.9

 15-24 years 20,892,354 21,307,916 415,562 2.0

 25-34 years 20,540,032 20,507,855 -32,177 -0.2

 35-44 years 20,353,203 20,585,375 232,172 1.1

 45-54 years 22,572,956 22,864,434 291,478 1.3

 55-64 years 18,632,438 19,027,576 395,138 2.1

 65-74 years 11,476,211 11,680,628 204,417 1.8

 75 years and over 11,575,510 11,303,305 -272,205 -2.4

Source: US Census Bureau Population Estimates Program 
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Table 3. Comparison of the National Resident Population by Sex, Hispanic Origin, and Race: July 1, 2010

Vintage 2010 

Postcensals 

Estimates 

Census 2010 Based 

Estimates

Differences

Numeric Percent

Both Sexes 309,050,816 309,349,689 298,873 0.1

 Non-Hispanic White alone 199,921,049 197,380,184 -2,540,865 -1.3

 Non-Hispanic Black alone 38,053,708 38,012,830 -40,878 -0.1

 Non-Hispanic AIAN alone 2,388,523 2,270,371 -118,152 -4.9

 Non-Hispanic Asian alone 13,992,198 14,738,414 746,216 5.3

 Non-Hispanic NHOPI alone 457,386 499,396 42,010 9.2

 Hispanic 49,545,523 50,810,213 1,264,690 2.6

Males 152,712,500 152,107,993 -604,507 -0.4

 Non-Hispanic White alone 98,389,162 97,062,393 -1,326,769 -1.3

 Non-Hispanic Black alone 18,171,245 18,116,542 -54,703 -0.3

 Non-Hispanic AIAN alone 1,179,537 1,119,358 -60,179 -5.1

 Non-Hispanic Asian alone 6,760,485 7,006,807 246,322 3.6

 Non-Hispanic NHOPI alone 230,140 251,820 21,680 9.4

 Hispanic 25,662,559 25,794,075 131,516 0.5

Females 156,338,316 157,241,696 903,380 0.6

 Non-Hispanic White alone 101,531,887 100,317,791 -1,214,096 -1.2

 Non-Hispanic Black alone 19,882,463 19,896,288 13,825 0.1

 Non-Hispanic AIAN alone 1,208,986 1,151,013 -57,973 -4.8

 Non-Hispanic Asian alone 7,231,713 7,731,607 499,894 6.9

 Non-Hispanic NHOPI alone 227,246 247,576 20,330 8.9

 Hispanic 23,882,964 25,016,138 1,133,174 4.7

Source: US Census Bureau Population Estimates Program