Illinois lost population in the 2020 Census and then it gained population after estimates of an undercount

The Census Bureau reported last week on states where populations were likely undercounted or overcounted. Illinois fell into the first camp:

Photo by Skylar Kang on

The U.S. Census Bureau originally found that Illinois lost about 18,000 people over the prior decade, which was the first time numbers showed Illinois’ overall population had declined since it joined the union in 1818. But after a follow-up survey — something that happens after each once-a-decade head count of the U.S. population — it discovered the state’s population figures had likely been undercounted…

“These latest numbers from the U.S. Census Bureau show that Illinois is now a state on the rise with a growing population,” Democratic Gov. J.B. Pritzker said in a statement. “ … I look forward to celebrating this development with all Illinoisans, including those who routinely bad-mouth our state.”…

The estimate that Illinois’ population was undercounted by 1.97%, or about 250,000, was the midpoint provided in the survey. The population could have been undercounted by as much as 440,000 people, or 3.43%, or as little as 65,000 people, or 0.51%, the survey showed.

I am interested in methodology and how small numbers can affect something seemingly simple like the number of people living in a state.

The Census Bureau puts much effort into getting the count right. This requires years of efforts leading up to the dicennial census, a lot of work in that year, and many resources put into all of it.

Losing 18,000 people over a ten year stretch is a small blip in a state that has over 12,000,000 residents. Similarly, gaining 250,000 people – the midpoint of the undercount estimate – is just a few percent of the total population. Either way, the Illinois population did not change all that much. It stayed relatively the same.

The real issue for Illinois and its leaders are the perceptions about population in Illinois and how Illinois compares to other states. Many agree: populations of state and communities should be growing for them to be deemed healthy. Population growth is good. In contrast, losing people is a sign of distress.

Similarly, even if Illinois grew by 250,000 people or lost 18,000 people, it matters how this compares to other places. Is Illinois more like Rust Belt communities and states or is it more like Sunbelt communities and states? Can Illinois be a shining light for population growth in the Upper Midwest? An so on.

It does not appear the overcount/undercount estimates will become official population figures so this story will likely continue in Illinois with subsequent Census estimates and political claims about population.

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