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.

Find Passel’s comparisons for states and the District of Columbia here and see other related reports at All Things Census.