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.
To read the full, original article click on this link: "Proofiness" Inherent in Population Milestones Such as "7 Billionth Person": Scientific American
