Wall Street Journal - June 29, 2007
Carl Bialik, The Numbers Guy**

The national debt clock, revived after a two-year hiatus now that deficits are piling up again, may soon need a new digit to keep pace. An AIDS clock, run by the United Nations, records more than 40 million sufferers world-wide. Visitors to the Bronx Zoo and the Museum of Natural History can watch the global population race toward seven billion as rain forest acreage dwindles. And the Bulletin of Atomic Scientists' Doomsday Clock has inched two minutes closer to midnight during its six decades of operation.

These count-up and countdown clocks pack information compactly into a compelling, even frightening, message. The trouble is that they're not very precise. No one knows how many people live on Earth, let alone how many are infected with HIV (which is what the AIDS clock actually is counting). The clocks generally run continuously and automatically -- until real numbers become available, at which time they can suddenly leap up or down.

Population clocks are a particularly popular device meant to suggest resources strained by exponentially growing numbers of people. The Census Bureau makes such a clock available on its Web site. This week, it shows a population of 6.6 billion. The American Museum of Natural History uses Population Connection's numbers to set its own count and hasn't updated its formula in years. In turn, Population Connection, a Washington, D.C., nonprofit that fights overpopulation, also hasn't updated its formula in years; its online clock stands at 6.62 billion this week. Meanwhile, the U.N. says there are 6.7 billion people in the world.

A lot of uncertainty surrounds figures for the number of people on Earth. "For roughly one-third of the countries of the world, we do not have reliable population information after 2000," says Gerhard Heilig, chief of the U.N.'s Population Estimates and Projections Section.

Three years ago, Bhutan's population was projected to be 2.26 million by 2007. But after a new census, the projected figure plummeted last year to 658,000. Even in countries, such as the U.S., that have more-regularly collected census data, there is little chance of a reliable count because of illegal immigrants.

The AIDS clock is even more questionable. Many sub-Saharan countries with the highest number of cases have little idea how many of their residents are infected. Estimates are based on extrapolating from a few sites. "They may or may not be representative of the HIV prevalence in the whole country," Dr. Heilig says.

The U.N.'s estimate of world HIV cases declined sharply in December, in part because of updated counts from various countries but also because of a genuine decline. Yet such changes don't always get reflected in these clocks. "We have just rechecked and recalibrated the clock a bit based on the most recently published figures," a U.N. Population Fund spokesman said in response to an inquiry from The Wall Street Journal last week. The clock's total, as a result, fell by more than 500,000.

The numerical foundation is firmer for the national debt clock, which since 1985 has been displaying both the total debt -- unadjusted for inflation -- and the average debt per family. The Museum of American Finance, scheduled to open on Wall Street later this year, is considering incorporating a similar clock in its exhibit halls.

New York real-estate developer Seymour Durst launched the clock in 1989. He died in 1995, but his family continued the tradition until the numbers started rolling backward in 2000. His son Douglas said the clock wasn't designed to do so. "We were trying to congratulate the Clinton administration on reducing the deficit by shutting down the clock." Two years later, the debt turned back upward, and the clock restarted. This week, it's at $8.89 trillion.

Today the clock is checked each week against federal numbers. "We're not as accurate as he was," Mr. Durst says of his father. "Ours is more of an estimate."

A forerunner of these electronic displays first appeared on the cover of the Bulletin of the Atomic Scientists in 1947. Martyl Langsdorf, an artist and the wife of atomic scientist Alex Langsdorf, designed the Doomsday Clock to symbolize the urgency and fears of the scientists, Kennette Benedict, publisher of the bulletin, says. The clock had no pretense of precision -- Ms. Langsdorf "put it at seven minutes to midnight because that's where it would look best in a design sense," Dr. Benedict says.

The scientists didn't expect to move the hand, but atomic tests by the U.S. and Russia pushed them to move the clock to two minutes before midnight in 1953. Since then, the minute hand has yo-yoed. Today it's at 11:55.

Paul Orselli, who designs exhibits for science and children's museums, says the best displays offer a tangible action viewers can take to address a problem. The Bronx Zoo's Congo exhibit, for example, allows visitors to direct their admission fees to conserving certain wildlife they just saw. On the other hand, if after seeing the population clock in the rain-forest exhibit you feel hopeless, he says, "then you're not going to take away a conservation message or an action message."

**Email me at numbersguy@wsj.com. Read daily commentary about numbers and join a discussion with readers at my free blog, WSJ.com/numbersguy
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