Estimates are based on reports (an application form) from each country. From these reports the UNPD demographers weigh census age, accuracy, and many other factors in adjusting their 2010 population estimates. They then calculate future population projections based on assumptions of fertility, income, education, health, and other social factors. Because these factors can not be predicted with certainty, the UNPD provides a high, medium, and low projection.
The U.S. Census Bureau uses their own sources, demographers, and formulas. Their world population projection is considerably more conservative than that of the United Nations. Mid 2013 they calculated the world population at 7.1 billion. By 2015 the US census bureau projects that world population will rise to 7.22 billion, nearly 2 years behind the UN projections.
* At Poodwaddle.com we have elected to use the United Nations medium projection for all population calculations, except where noted. Death rates, too, are calculated from UNPD reports. Compare this with the similar (but not identical) world death rate on the Mortality pages where the World Health Organization projections are used. Although we could have interpolated the data to make one agree with the other (essentially fudging the numbers), we chose, instead, to leave this disparity and keep source accuracy.
SOURCES: " + 'UN Population Division | US Census Bureau