Illinois lost residents over the last decade. But, different Census estimates at different times created slightly different stories:

Those estimates showed Illinois experiencing a net loss of 9,972 residents between 2013 and 2014; 22,194 residents between 2014 and 2015; 37,508 residents between 2015 and 2016; about 33,700 residents between 2016 and 2017; 45,116 between 2017 and 2018; 51,250 between 2018 and 2019; and 79,487 between 2019 and 2020…

On April 26, the U.S. Census Bureau released its state-by-state population numbers based on last year’s census. These are the numbers that determine congressional apportionment. Those numbers, released every 10 years, show a different picture for Illinois: a loss of about 18,000 residents since 2010.

What’s the deal? For starters, the two counting methods for estimated annual population and the 10-year census for apportionment are separate. Apples and oranges. Resident population numbers and apportionment population numbers are arrived at differently, with one set counting Illinois families who live overseas, including in the military, and one not.

Additionally, the every-10-years number is gathered not from those county-by-county metrics but from the census forms we fill out and from door-to-door contacts made by census workers on the ground.

The overall story is the same but this is a good reminder of how different methods can produce different results. Here are several key factors to keep in mind:

1. The time period is different. One estimate comes every year, one comes every ten years. The yearly estimates are helpful because people like data. That does not necessarily mean the yearly estimates can be trusted as much as the other ones.
2. The method in each version – yearly versus every ten years – is different. The decennial data involves more responses and requires more effort.
3. The confidence in the two different kinds of estimates is different because of #2. The ten year estimates are more valid because they collect more data.

Theoretically, the year-to-year estimates could lead to a different story compared to the decennial estimates. Imagine year-to-year data that told of a slight increase in population while the ten-year numbers provided a slight decrease in population. This does not mean the process went wrong there or in the narrative where the yearly and ten-year estimates agreed. With estimates, researchers are trying their best to measure the full population patterns. But, there is some room for error.

That said, now that Illinois is known as one of the three states that lost population over the last decade, it will be interesting to see how politicians and business leaders respond. I can predict some of the responses already as different groups have practiced their talking points for years. Yet, the same old rhetoric may not be enough as these figures paint Illinois in a bad light when population growth is good in the United States.