(Editor's note: This article was published first at Illinois Policy Institute).
On May 19, the U.S. Census Bureau released state-level results for their Post-Enumeration Survey. The survey estimates Illinois’ household population was undercounted by 1.97% during the 2020 official Census.
Gov. J.B. Pritzker used that information to claim Illinois welcomed over 250,000 new residents.
But the Census Bureau’s Population Estimates Program tells a different, consistent story. It has estimated Illinois population declines each year since 2014, including in 2021 when its numbers showed a record-setting loss of 114,000 residents.
Two separate estimates and a head count each yielding different numbers complicate this simple question: “Are Illinoisans leaving for greener pastures?” That is, unless you have a data analyst to help sort through it, which the Illinois Policy Institute is happy to provide. Stay with us.
What is really going on with Census data?
To accurately decipher the various estimates of population levels and changes it is important to have a fundamental understanding of what each program from the Census Bureau does, and how it is intended to be used.
Let’s start with the official decennial census, the most recent of which was the 2020 census count. Each decennial census is an count of the U.S. population on April 1 of the reference year. The population is determined based on physical responses from each household and the results are used to determine representation in Congress and the allocation of some federal spending. These counts include those living in group settings such as college dorms, nursing homes and prisons. The 2020 official Census count estimated Illinois’ population to be 12,812,508 as of April 1, 2020.
After each decennial census, the Census Bureau conducts a Post-Enumeration Survey (PES). This survey is intended to measure the accuracy of the official census count by independently surveying a sample of the population. This survey is conducted at the household level and does not include populations living in group settings. The 2020 Post-Enumeration Survey determined that Illinois’ household population was undercounted by 1.97% in the official census count. That means on April 1, 2020, the household population according to the PES was 12,792,000 rather than the household population of 12,540,000 reported in the official count. Again, those in group settings were not included in that number, so adding an estimate of those folks in would imply Illinois’ total population on April 1, 2020, was 13,065,000.
For time periods between each decennial census count, the Census Bureau’s Population Estimates Program, or PEP, produces population estimates based on the results of the most recent decennial census. PEP annually uses current data on births, deaths and moves to calculate population change since the most recent decennial census. It produces a time series of estimates of population, demographic components of change and housing units. Because these estimates use the latest decennial census count as their starting point, PEP estimates of Illinois’ population on April 1, 2020, match the official count of 12,812,508.
This program is also intended to track changes in the population based on: federal vital statistics data from the National Center for Health Statistics and Federal-State Cooperative for Population Estimates; domestic migration data from the Internal Revenue Service, Medicare, Social Security Administration and the Census Bureau’s Demographic Characteristics File for all ages; and international migration data from the American Community Survey, Puerto Rico Community Survey and the Defense Manpower Data Center. It is possible for estimates of births, deaths and net migration to be correct in these estimates, even if baseline population counts from the official census count differ from other estimates.
The bottom line
The Census Bureau conducts many different surveys and programs in order to bring the most accurate and timely data possible to stakeholders in government, research, media and the general public. The Census Bureau computes estimates to both check the accuracy of each census count and to provide annual data between each decennial census. The results of one Census program or survey do not invalidate another; instead, these results should be seen as pieces in a mosaic that taken together give a full picture of what is happening to the population of Illinois.
While there have been discrepancies between the Census Bureau’s estimates of the population and their official decennial Census count, there are conclusions to be drawn from the big picture offered by viewing the Census Bureau products together. Mainly:
- Illinois’ population is higher than previously believed. The 2020 Census and PES have confirmed Illinois’ population is higher than previous estimates based on the 2010 Census. It is likely increased outreach efforts and changes to methodology – such as the option to respond to the 2020 Census online – resulted in a more accurate count this time than a decade earlier.
- Illinois’ population is in decline. Despite 2020 Census counts that were higher than estimates based on the 2010 Census count, estimates of population change were likely accurate when they showed decline. Evidence of the downward trend includes the fact that even after the 2020 Census count reset the baseline, the Census Bureau’s PEP estimated the largest population decline in Illinois history for 2021. Other estimates of migration from the IRS and multiple moving companies have found more people continue to move out of Illinois than into the state. Because birth and death rates virtually offset each other, domestic migration is the main driver of Illinois population change.
- Confusion surrounding estimates of population levels and change could be fixed through better integration of Census Bureau products. During the Census Bureau’s webinar on the PES results, representatives said this was an ongoing effort.
Rather than trying to spin Census data for political gain during a reelection campaign, Pritzker should focus on reforms to address Illinois’ $313 billion pension debt that is eating state revenues and crowding out valuable services for residents. It is also forcing them to pay the highest taxes in the Midwest.
Having fewer people to pay those taxes, and pretending they are still here, makes the problem worse.