 Population estimates cannot be considered accurate to more than two decimal digits;
for example, the world population for the year 2012 was estimated at
7.02, 7.06 and 7.08 billion by the United States Census Bureau, the Population Reference Bureau and the United Nations Department of Economic and Social Affairs, respectively, corresponding to 
a spread of estimates of the order of 0.8%.As a general rule, the confidence of estimates on historical world population decreases for the more distant past.Robust population data exists only for the last two or three centuries.