The 2000 census missed a little more than 1 percent of the nation's population, according to follow-up surveys conducted by the U.S. Census Bureau. One of the biggest contributors to the error was a surge of undocumented immigrants to the United States in the late 1990s. Nevertheless, the undercount was far less than the bureau's researchers and others expected it to be.
The census forms filled out by U.S. residents early in 2000 recorded more than 274.6 million people. The Census Bureau's initial demographic analyses, conducted separately from the census, predicted there would be a population of about 281.4 million, or nearly 7 million more people than were actually recorded.