Population Basics Part 1

Data and Metrics

Dear Pandemic is excited to introduce Nerdy Guest Michal Engelman, PhD. Dr. Engelman teaches and studies population health and aging at UW-Madison’s Center for Demography and Ecology. She joins us today to talk some basics about population. She’ll be back for part 2 on Thursday!

Q: I’ve never heard the word population so much in my life, and I’ll be honest… I don’t really know what that is. What is a population?

If we look “population” up in a dictionary, the first definition is “the whole number of people… in a country or region.” The second is a bit broader: “the total of individuals occupying an area or making up a whole.” Seems simple enough, right? We start counting, and when we finish, we’ve got a number, and that’s our population.

But it gets complicated fast. Let’s take the United States as an example. How do we define its population? We could go door to door counting people; that’s the goal of the U.S. Census. But should the U.S. Census count all people in the country, or just citizens, or those who own property, or who are a particular race and/or gender? All of these have been debated at some point in our country’s history.

What’s Wrong with the Census?

And speaking of history: as births, deaths, and migration add and subtract people, our population changes. So we have to decide how often we’re going to count, too.

There’s power in numbers. How we decide what whole is being counted, and who gets included in it, and when: these are all social and political decisions.

Once we decide on the whole, we can use it as the denominator in calculating population rates or averages. For example, is 20 COVID-19 cases (or deaths) per day a little or a lot? Each case is, of course, a person who matters enormously. Assessing the pandemic’s overall impact – and the appropriate response to it – depends a lot on whether each case occurs in a population of 100 people or 100,000 people. Comparing the total number of cases (the numerator in our prevalence rate) to the total population (the denominator) helps us see the bigger picture.

But sometimes we also need to be able to see the details in the picture, to understand what’s generating the total number of cases. What if a bunch of cases in a given population are concentrated in a particular state, town, or neighborhood? Or among workers in a particular industry? Or among members of a particular racial or ethnic group? If you only count the whole, you miss out on what’s happening to specific groups of people.

In some cases defining such subpopulations – by traits like race, gender, or class – may be even trickier than defining the whole population. It might not seem like a public health question. But it turns out that defining subpopulations matters a lot for how we act on public-health problems, and especially how we do that fairly as well as effectively. So understanding how populations work is crucial to stopping, slowing, or otherwise controlling the pandemic. And that will be the subject of my next post (you’ll have to wait until Thursday!)

Link to original FB post