# Happy World Population Day!

Happy World Population Day from Eco18! At this very moment, the Earth holds about 7.7 billion people, distributed in clusters across the globe. If you want to be overwhelmed by the number of people being born each day, just click here and watch the numbers rise. But how do scientists and mathematicians calculate this number? What is the margin of error? How is it even possible to estimate the population of the entire world?

### What is population?

The population is measured in two ways: population size and population density. The size is determined by the number of people in that area and the density is how many people are within a certain area or volume. For example, New York City, NY has a population of 8.55 million people and a population density of 27,962 people per square mile as of 2016. For comparison, Washington D.C. contains 670,000 people and has a population density of 10,983 per square mile.

### Why is the U.S. Decennial Census Important?

The census takes place every ten years, with the last census occurring in 2010. In 2010, New York City had 8,175,133 people but as mentioned earlier, by 2016, the city had increased its population by almost 375,000 people! The census calculates how many people are in each area by asking household size, age, sex, race, and more. This number is then used to determine voting district lines, how many seats a state gets in the House of Representatives, demographic information by region, and how almost \$400 billion dollars of federal funding will be allocated for public health, education, roads, and much more. In the words of the Census Bureau, “The census tells us who we are and where we are going as a nation, and helps our communities determine where to build everything from schools to supermarkets, and from homes to hospitals.” This year, the Trump presidency is trying to add a question on citizenship status too, causing massive unrest between political parties about the motives of these efforts.

### How are these numbers calculated?

In the past, the majority of people have mailed in their questionnaires to be counted, and those who did not mail in were canvassed by census employees who physically visited the neighborhoods to record census answers. This can be quite an expensive ordeal and the 2010 census cost over \$12 billion dollars to record. However, in 2020 the first ever digital census will be sent out, hopefully increasing access and response to the census. The hope is that this will decrease the margin of error or the amount of “possible variation of an estimate around the population value”, and it has already been successfully done in Jordan. All eyes will be on cybersecurity in the coming year to see just how safe this new form of census-taking will be.

### Does every country have a census?

At the moment, only 100 countries complete a census and the US Census Bureau is working with USAID to expand this number. As the map below details, this effort is spreading but obviously does not expand across all countries. (Keeping in mind that the USA does have a census but does not count as “receiving aid” because the US Census Burea is a local agency).

Ultimately, the world population is calculated by the census data that is attainable and by estimating the remaining countries using their last available population data and factoring in their birth rate and their death rate. You can find out how each country’s population size is calculated here. Despite the need for approximation, the margin of error for world population is expected to be very low, but still, an exact calculation is impossible.

### Why is calculating the world’s population important?

Beyond the obvious reasons for the importance of having an accurate number for population size and density for all types of scientific research, there is a constant debate of the existence of carrying capacity for the Earth. How many people can the Earth’s resources support? How do we calculate how many resources are required by human and non-human life for survival? What are the effects of overpopulation on our quality of life? Some of these same questions are what inspired the creation of World Population Day by the United Nations in 1989.

This year the United Nations is focusing on gender equality and reproductive rights. The Executive Director of the UN World Population Fund, Natalie Kanem, said in an article for the UN, “It’s time to usher in a world where promises made are promises kept, and reproductive rights and choices are a reality for all. This is the world we all want and can have if we join together in Nairobi and beyond, with concrete commitments and far more resources to complete the journey we began 25 years ago”. This November, a conference to expand and solidify promises made at the Cairo International Conference on Population and Development to promote gender equality will be convened in Nairobi. To stay updated with the plans of this conference, follow the United Nations on Twitter here