January 12, 2011

State Population Estimates and Census 2010 Counts: Did They Match?

How well did the Census Bureau’s population estimates for the first decade of the 21st century match the actual counts from the 2010 Census?  The short answer: Pretty well for the nation, and for all but a handful of states.

The accuracy of these population estimates is important because the numbers, which are released each year in between the once-a-decade census counts, are the basis for distributing billions of dollars in federal funds and are the denominators for rates used in some federal surveys. Unlike the census, which counts people directly, the estimates are assembled using government data, including birth and death certificates, immigration estimates and tax-return statistics on people who changed residences.

As Census Bureau Director Robert M. Groves pointed out in a news conference today, it greatly increases confidence in the census count if population numbers that are derived using different methods are similar. Groves said it reflects well on 2010 Census accuracy that the 2010 Census count for the nation, 308,745,538, was close to the bureau’s national population estimate of 308,977,944 on Census Day (April 1, 2010). The 2010 national count also matched up well with population estimates the bureau released last month using demographic analysis, an alternative measurement technique that uses a method somewhat similar to that used in the population estimates.

Digging into the estimates data, Pew Research Center demographer Jeffrey Passel has analyzed how the bureau’s population estimates for states compare with the official 2010 Census counts for states. He began with state estimates for July 1, 2009 (the latest available), and projected them forward to Census Day based on average growth rates for each state for 2007-2008 and 2008-2009. (This method produces a slightly different total for the nation than the Census Bureau computed, 309,081,328.)

According to his analysis, for most states, the census count and projected population estimates are quite close, showing a difference of less than 1%. In only six states—Arizona, Georgia, Hawaii, North Dakota, Utah and Wyoming—did the count and estimates differ by 2% or more. Arizona had the largest numerical and percentage difference. The population estimates had about 294,000 more people living in the state on Census Day than the 2010 Census counted, a difference of 4.6% from the census count.

Compare that with the 2000 Census, when there was a notable gap between the population estimates and the census count.  The Census Bureau’s population estimates for April 1, 2000 (274 million) fell short of the census count for that day (281 million) by 2.44% and nearly 7 million people. The bureau’s demographic analysis in 2000 also fell short of the 2000 Census population totals. Even though the 2000 Census is believed to include a slight overcount of the total population, the difference among the numbers was troubling. It was largely attributed to underestimates of immigrants throughout the decade, so bureau officials went back to the drawing board to make improvements in how they counted both legal and unauthorized foreign-born residents.

Passel’s comparisons for states and the District of Columbia are shown below:

2010 Census 2010 Estimate1 Difference % Difference
308,745,538 309,081,328 -335,790 -0.11%
Alabama 4,779,736 4,735,563 44,173 0.92%
Alaska 710,231 704,646 5,585 0.79%
Arizona 6,392,017 6,685,751 -293,734 -4.60%
Arkansas 2,915,918 2,907,391 8,527 0.29%
California 37,253,956 37,241,678 12,278 0.03%
Colorado 5,029,196 5,095,104 -65,908 -1.31%
Connecticut 3,574,097 3,529,479 44,618 1.25%
Delaware 897,934 892,839 5,095 0.57%
District of Columbia 601,723 604,709 -2,986 -0.50%
Florida 18,801,310 18,636,538 164,772 0.88%
Georgia 9,687,653 9,942,567 -254,914 -2.63%
Hawaii 1,360,301 1,302,132 58,169 4.28%
Idaho 1,567,582 1,563,664 3,918 0.25%
Illinois 12,830,632 12,959,908 -129,276 -1.01%
Indiana 6,483,802 6,452,250 31,552 0.49%
Iowa 3,046,355 3,018,862 27,493 0.90%
Kansas 2,853,118 2,835,121 17,997 0.63%
Kentucky 4,339,367 4,336,022 3,345 0.08%
Louisiana 4,533,372 4,536,419 -3,047 -0.07%
Maine 1,328,361 1,318,674 9,687 0.73%
Maryland 5,773,552 5,724,154 49,398 0.86%
Massachusetts 6,547,629 6,629,338 -81,709 -1.25%
Michigan 9,883,640 9,939,491 -55,851 -0.57%
Minnesota 5,303,925 5,294,646 9,279 0.17%
Mississippi 2,967,297 2,963,436 3,861 0.13%
Missouri 5,988,927 6,017,026 -28,099 -0.47%
Montana 989,415 981,743 7,672 0.78%
Nebraska 1,826,341 1,806,747 19,594 1.07%
Nevada 2,700,551 2,671,953 28,598 1.06%
New Hampshire 1,316,470 1,327,298 -10,828 -0.82%
New Jersey 8,791,894 8,734,792 57,102 0.65%
New Mexico 2,059,179 2,025,262 33,917 1.65%
New York 19,378,102 19,586,160 -208,058 -1.07%
North Carolina 9,535,483 9,502,784 32,699 0.34%
North Dakota 672,591 650,118 22,473 3.34%
Ohio 11,536,504 11,550,843 -14,339 -0.12%
Oklahoma 3,751,351 3,715,559 35,792 0.95%
Oregon 3,831,074 3,861,064 -29,990 -0.78%
Pennsylvania 12,702,379 12,635,757 66,622 0.52%
Rhode Island 1,052,567 1,052,535 32 0.00%
South Carolina 4,625,364 4,613,808 11,556 0.25%
South Dakota 814,180 818,221 -4,041 -0.50%
Tennessee 6,346,105 6,343,217 2,888 0.05%
Texas 25,145,561 25,146,986 -1,425 -0.01%
Utah 2,763,885 2,831,392 -67,507 -2.44%
Vermont 625,741 622,248 3,493 0.56%
Virginia 8,001,024 7,944,618 56,406 0.70%
Washington 6,724,540 6,740,619 -16,079 -0.24%
West Virginia 1,852,994 1,823,006 29,988 1.62%
Wisconsin 5,686,986 5,674,867 12,119 0.21%
Wyoming 563,626 552,323 11,303 2.01%
  1. 2010 Estimate is a projection to April 2010 of the Census Bureau’s July 1, 2009 estimate, using the average growth rate for the previous two years. Source: Pew Research Center analysis of U.S. Census Bureau data.