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How Do You Count 7 Billion People?

The United Nations Population Division (UNPD), which keeps track of the world population, projects that the world's human population will hit 7 billion on Halloween Day 2011. Admittedly, that is just an estimate: There's no way to know exactly how many people are alive at any given moment, and the true date that humanity's ranks will surpass 7 billion could come in the weeks or months before or after Oct. 31. Nonetheless, the UN's guess is the best there is.

How do they make it? By synthesizing a mind-boggling array of data.

According to a chief analyst in the UNPD, its population estimate relies on fertility, mortality and migration information gathered by government censuses, independent demographic and health surveys, vital registers (official birth and death records), the World Health Organization, the UN High Commission on Refugees, and academic studies. UN analysts revise their country and world population curves every five years to account for any new data gathered by those entities since their last revision; they completed the current population projections earlier this year. [Center of U.S. Population Moves West]

Censuses — population statistics gathered periodically by governments — only go so far. "The uncertainty in census data is very high: in the range of 2 to 3 percent in most countries," the UN analyst, who asked not be named, told Life's Little Mysteries. That range might not sound very large, but for a country like China, which has a population of 1 billion, that means a 40-to-60-million-person error.

"In other countries, census data is very limited. Either there hasn't been a census for decades, or the census that has been taken is disputed," the analyst said. In many war-torn areas of the world, for example, it's logistically impossible to accurately count people. Some gaps in census data pertaining to developing countries get filled in by independent organizations such as Demographic and Health Surveys (DHS), a nonprofit based in the United States, but in many countries, uncertainty remains.

Furthermore, if you're trying to predict the population of a given country at some future time — even if that future time is just the publication date of your report — you obviously need to know the fertility, mortality and migration rates in the country. Are people having lots of kids or not? Are they dying young? Are they going elsewhere? [Why Do We Have Sex?]

The DHS, as well as UNICEF, the World Health Organization (WHO), and the UN Commission on Refugees all contribute to gathering information about fertility, infant and adult mortality rates and migration rates around the world.

"We also use data collected by WHO concerning HIV/AIDS prevalence. This is because, in a number of countries, you cannot estimate mortality independent of the AIDS epidemic," the UN Population Division analyst said.

These various data sources enable the UN Population Division to establish the growth rates of each age bracket in each country, and determine the overall upward trend in the world population over time. (It extends the curves all the way to the year 2100.) "You typically get a data cloud — a range of data points — rather than a simple line, and it's very complicated to come up with estimates and put a line through this data cloud." In short, who knows what the future holds? The UN picks a single projection from among a huge range of possibilities.

UN statistics show that the population will reach 10 billion by 2100, if worldwide fertility converges to "replacement level" by that time — if people have only enough children to replace themselves.

"Many people are not aware that, with our projections, we assume that people change their reproductive behavior. We assume that people in those countries where fertility is still very high will change and have significantly less children," the analyst explained. "But this is an assumption based on experience in other countries; this is by no means guaranteed. It could be that people don't want to reduce their fertility for some reason. If the fertility would not decline in the world, if it would stay at the same level as 2010, we would have 27 billion people in 2100."

Africa, not Asia, is the biggest concern for population analysts, he said. "Essentially all population growth between now and the end of the century is happening in Africa. It's not Asia; in fact, fertility in China is below the replacement level. It's Africa, because there are many countries that have very high fertility. There are many young people there, so it's a very 'high momentum' population."

This story was provided by Life's Little Mysteries, a sister site to LiveScience. Follow Natalie Wolchover on Twitter @nattyover. Follow Life's Little Mysteries on Twitter @llmysteries, then join us on Facebook.