In 1956 the UN's projection for the year 2000 was 6.3 billion people. We now know that the real number for the year 2000 will be very close to 6.06 billion, so the error made in 1956 was 3.6 percent. In 1963 they made a new projection of 6.1 billion for the year 2000. That represents an error of 1.2 percent. According to Bongaarts, an error of a few percentage points over such a long period represents an impressive record.
Several factors contribute to the ability to gauge future population so well.
Population doesn't turn over very fast. And demographers have become quite proficient at predicting general trends in birth and death rates. What the future holds Projections offer a snapshot of what the world might be like in 2050. Nearly all the population growth is projected to occur in less-developed countries, where resources are the most limited. In the wealthier developed nations, little change in population growth is expected and some countries will experience population declines.