Every few years, public officials and the news media perform a ritual form of "disestimation" when a population clock reaches a big milestone. Population experts at the Census Bureau and around the world are constantly estimating the populations of each nation. Their estimates are pretty good, predicting when, say, the world's population reaches six billion—they might even be able to guess when the six billionth person is born to within a few hours. That's about as good as any possible measurement of population can get. Populations constantly fluctuate, with people dying and being born at irregular intervals, often far from the eyes of people who count such things, so it's impossible to know at any given moment the true number of people alive on earth.

Nevertheless, on October 13, 1999, as flashbulbs popped around him, UN secretary-general Kofi Annan held a young Bosnian boy, welcoming him into the world as the six billionth person on earth. (The UN insisted that Annan's presence in Sarajevo was a complete coincidence. It was just a lucky break that the six billionth person was born in the city where Annan happened to be visiting.)

There's no way that anyone could pinpoint which baby became the six billionth person living on earth. The uncertainties in measurement are simply too huge. You wouldn't know, probably to within a couple of thousand, whether a baby is number 6,000,000,000 or 5,999,998,346 or 6,000,001,954. Only by disestimating, by ignoring the uncertainty in population numbers, could anyone claim to know for certain who was the six billionth living person.

Disestimation is the act of taking a number too literally, understating or ignoring the uncertainties that surround it. Disestimation imbues a number with more precision than it deserves, dressing a measurement up as absolute fact instead of presenting it as the error-prone estimate that it really is. It's a subtle form of proofiness: it makes a number look more truthful than it actually is—and the result can be as silly and meaningless as a museum guide's 65,000,038-year-old dinosaur.

Yet at every population milestone, world officials and the news media go through the same bizarre pantomime. In 2006, the Chicago Sun-Times declared a local baby—Alyzandra Ruiz—to be the 300 millionth resident of the United States. (They cleverly jumped the gun on everybody, making the arbitrary call almost an hour before the official Census Bureau population estimate reached the 300 million mark.) And when the world population reaches seven billion, officials will declare some lucky baby to be the seven billionth living person, completely indifferent to the fact that it's a lie.

A disestimate has its origin in a real, meaningful, good-faith measurement—the problem is that we don't take the resulting number with a big enough grain of salt. It's a rather subtle problem. As a result, disestimates can be difficult to spot. Once a disestimate is believed by the public, it can be devilishly hard to debunk.

Failure to recognize the inherent limitations of a measurement can be extremely dangerous, because it can potentially create an authentic-sounding number that is in fact far removed from the realm of truth.

Reprinted by arrangement with Penguin Books, a member of Penguin Group (USA) Inc., from Proofiness: The Dark Arts of Mathematical Deception by Charles Seife. Copyright © 2011 by Charles Seife.