As of Oct. 12, there were 6 billion people on Earth.

Or maybe not.

The date might have been July 19. Or was it in April?

"You can't say exactly when," Corrie Shanahan concedes, though that has not stopped her agency, the United Nations Population Fund, from declaring Oct. 12 as the official "Day of Six Billion."

The date was chosen as much for convenience as accuracy, Shanahan said. Demographers with the United Nations' Population Division narrowed down the 6 billion milestone to some time in early October. But that vague estimate lacked punch in proclamations, so officials at the Population Fund, a U.N.-sponsored agency similar to UNICEF, chose Oct. 12.

"It's not entirely arbitrary, but it is definitely symbolic," Shanahan said. "We can't say it's that day, or even narrow it down to within a day or two."

They're even further off than that, according to the U.S. Census Bureau's International Programs Center, which projected July 19 to be the 6 billion day.

Both agencies use the same census figures from around the world. But different methods of analyzing the data, including the Census Bureau's assumption of a slightly higher fertility rate, help explain the three-month discrepancy, said Census Bureau demographer Tim Fowler.

Neither figure should be mistaken for hard fact. Putting a number on the planet's bustling multitudes is less like toting up coins in a piggy bank and more like counting bees in a hive.

Any current estimate of world population is actually a projection based on census figures up to a decade old.

"We really don't have any data for 1999," Fowler said. "For a lot of countries, the most recent information is from around 1990."

Demographers run census figures through a "smoothing" process, trying to correct data-collection errors particular to each country. Then they extrapolate into the future from the date of the census, using each nation's estimated fertility, mortality and migration rates.

Add up all the countries, and the result is a world population figure that's stronger on precision than accuracy.

"We talk about the global village, but we don't have as much information as we sometimes think we have," Shanahan said. "It says something about how vast the Earth still is, and how immeasurable some things are."

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