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The 300 Millionth AmericanHow do we find him?

By Daniel Engber
Updated Monday, Oct. 16, 2006, at 6:46 PM ET

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The population of the United States is about to hit 300 million. According to the U.S. Census Bureau, the milestone will be reached Tuesday, at 7:46 a.m. EDT.* How did they know when it would happen?

It's a guess. The bureau collects complete census numbers only once every decade, but it puts out population estimates much more often. To get a crude sense of the population between milestone census years, the demographers look at monthly birth and death rates supplied by the National Center for Health Statistics. These come from registered birth and death certificates, which are thought to represent more than 99 percent of all births and deaths.

Given these data, researchers at the bureau can make estimates about the rate of population growth in the next month, or the next year. Once they've made a guess about the total growth for October, the researchers can divide by 31 to get a daily rate, and then by 24 to get an hourly rate. Since they assume that the distribution of births and deaths is uniform throughout the month, they can keep dividing until they have estimates for fractions of a minute. For this month, they're expecting one birth every seven seconds and one death every 13 seconds. At that rate, we'll hit 300 million just before rush hour on Tuesday morning.

The figure of 300 million also reflects an estimate for the number of foreigners who move into the United States every month, and the number of native-born Americans who leave. The latter number also includes members of the U.S. military who leave the country for posts overseas.

The military adjustment is easy—the Census Bureau can grab the deployment statistics from the Department of Defense. It's much harder to get a handle on international migration. In the 1990s, this information came from government records on legal immigration. To get stats for migration from Puerto Rico, the bureau checked the logs of flights to and from the island and tried to determine the net influx of passengers.

These methods weren't too effective. In 2000, the bureau checked the new population total from the census against the guesses it had made throughout the 1990s—the running totals were off by almost 7 million. To correct this discrepancy, demographers started looking at new ways to estimate population. They also worked on ways to collect meaningful population data between big census years. Over the last decade, the bureau has developed an annual "American Community Survey," which now gathers census-type information from a sample of about 250,000 addresses per month.

Got a question about today's news? Ask the Explainer.

Explainer thanks Signe Wetrogan of the U.S. Census Bureau.

Correction, Oct. 17, 2006: Due to an editing error, the original version of this piece said the milestone would be reached at 7:46 a.m. Eastern Standard Time. In fact, it was 7:46 a.m. Eastern Daylight Time. (Return to the corrected sentence.)

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Daniel Engber is an associate editor at Slate. He can be reached at .
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