According to the U.S. Census Bureau, the world's population reached 6 billion people on June 19, 1999. The Bureau operates a population "clock" that shows the population of the world changing second by second. How is this possible? How can anyone possibly know the exact population of the world at any given second? Of course, the answer is that no one can know the world population with the degree of accuracy implied by the population clock.

And yet accurate projections of future population growth are indispensable to world leaders who must plan and set policies for health care, education, housing, energy, transportation, food and water delivery, environmental protection, disaster preparedness, and national defense. The collection and continual updating of data from the nations of the world and the use of sophisticated mathematical models provide projections that are as accurate as is currently possible.

Making Projections

The 6 billion mentioned...