Throughout eight Senate redistricting hearings, much has been made about the type of data Illinois lawmakers will use to help redraw the state’s political boundaries this year.

Leading Democrats have yet to make a final decision about which data will be used to draw state lines. But the party’s goal of drawing new maps before June 30 will mean this year’s redistricting process will be the first time the state does not utilize the U.S. Census Bureau’s decennial data, which is not expected to be released until the fall.

Democrats say their decision to consider using a different data set from the decennial data is largely due to the delay. Instead, they say alternative data sets can help move the redistricting process along, pointing to a broader goal of ensuring the redrawn maps reflect the state’s diversity.

Meanwhile, Republicans and a growing chorus of witnesses who have testified during the initial Senate meetings have expressed repeated concerns about Democrats’ plans and what they call the dangers of using alternate data.

And although every state in the country must redraw its political maps in the coming months, the debate over data is putting Illinois at the forefront of the discussion.

Differences in data sets

Unlike some states, Illinois’ Constitution is silent on the type of data that legislators may use for redistricting.

Aside from decennial data, the U.S. Census Bureau has several different data sets that state leaders are considering using for redistricting, including American Community Survey data and population estimates.

Population estimates represent annual adjustments of the 2010 Census data that incorporate estimated deaths, births and population shifts.

The American Community Survey data has two types: one-year and five-year estimates.

The one-year estimate, however, is likely unable to meet requirements for redistricting because it only provides insight on municipalities with 65,000 or more residents.

The five-year estimates have a narrower focus, going down to the block level, while providing a more up-to-date understanding of changes in communities over time.

Census officials conduct the one- and five-year estimates by sending out surveys, asking people to provide information on everything from education and employment to internet access in an attempt to provide officials a better understanding of their communities. Unlike the decennial Census, which attempts to reach every person in the United States, the American Community Survey is sent to a sample of about 3.5 million addresses each year.

The various alternate data sources are less precise than the decennial census.

The decennial census is conducted through enumeration, a process that involves significant outreach in an attempt to ask people where they live and how many people live in a given dwelling.

Decennial data also goes down to what’s called the “Census block” level, the smallest unit of geography.

“A Census block for the Census Bureau could be the on ramp to an interstate,” Ben Williams, an elections and redistricting policy specialist with the National Conference of State Legislatures, said in a recent interview with The Daily Line.

Population estimates are typically adjusted based on larger political subdivisions, such as counties, Williams said.

He said the American Community Survey’s five-year estimates report data on the Census block level but not quite to the granular level of the decennial Census.

Disagreements over data

Earlier this month, Gov. JB Pritzker expressed support for lawmakers’ plans to use alternate data.

Asked on March 24 how voters can trust a map that doesn’t rely on decennial data, Pritzker said, “Because we know an awful lot about the data of population movement and so on.”

Pritzker said it wasn’t the state’s “fault” that the Census Bureau wasn’t prepared, laying blame on former President Donald Trump’s administration and the COVID-19 pandemic.

“We have deadlines to meet and so there’s great data out there to be used. I know the legislature is looking at that data now,” Pritzker said, minutes before he received his COVID-19 vaccination.

Related: Pritzker vaccinated at Illinois State Fairgrounds

The governor said it was “reasonable” for lawmakers to look at different data sources, which Pritzker said have been “used by the Census Bureau itself.”

The Census Bureau, however, uses American Community Survey data population estimates to provide a snapshot of states, not to redraw political boundaries.

Although Pritzker did not indicate any concern about using alternate data, representatives from a host of organizations, including CHANGE Illinois, Common Cause Illinois and others, have offered warnings in recent hearings.

Related: Redistricting witness warn against using alternate Census data, call for greater engagement among minority groups

During a Tuesday Senate Redistricting hearing, Griselda Vega Samuel, an attorney with the Mexican American Legal Defense and Education Fund, said American Community Survey data can be used to look at “general trends.” But she warned that the data is “only a small sampling” of the total population, leading to a “underrepresentation” of the Latino community.

“When you’re drawing maps that are going to affect the entire state for the next ten years, it should be that you’re getting block-level data,” she said. “When it comes to the final map, they should be using the best data available, and that is the Census data, as we have for so many, so many years.”

Democrats have tried to allay concerns about their plans to use different data, saying for now the party will use American Community Survey data on its senate redistricting website, which includes a portal for the public to create their own maps.

On Tuesday, Sen. Omar Aquino (D-Chicago), who chairs the senate redistricting committee, announced the public portal was live. The Daily Line’s initial attempts to use the portal were stymied, after a verification link sent from the website via email was deemed “invalid.”

Related: Republicans renew push for independent redistricting group as Senate hearings continue

States taking different approaches

In some ways, the debate over data during this year’s redistricting process hearkens back to the early 2000s.

“Not only will Republicans and Democrats be elbowing for advantage in the way new district lines get drawn, but for the first time state leaders almost surely will be bickering about the very numbers on which those lines will be based,” according to a Tribune story from February 2000.

The story noted how Republicans “generally favor a simple head count, while Democrats argue for adjusting the numbers with a statistical sample to make up for people believed to have been missed by the county — in large measure minorities and immigrants who tend to support Democrats.”

Williams said the data debate during the 2001 redistricting process largely had to do with questions about whether sampling was a valid statical measure to resolve “holes” in decennial data. The U.S. Supreme Court said it was acceptable, he said.

Williams said since the 1990s, states have “generally accepted” the Census Bureau’s decennial data as the default source for redistricting. Most of the time, states have the data early, giving them flexibility.

“It’s a very high-quality data set that is free,” Williams said.

But the delayed data this year has forced states to adjust, including in Illinois, which is one of five states that have a redistricting deadline set in the state’s Constitution before June 30, according to the National Conference of State Legislatures.

Faced with the delayed Census data and such early deadlines, some states have taken new approaches.

Oklahoma plans on using the American Community Survey’s five-year estimates to draft lawmakers’ districts within 90 days of the end of their current legislative session.

Idaho state legislators have also said they will use Census population estimates to redraw their boundaries. Williams said that’s possible because Idaho splits up “very few” of its 44 counties during redistricting.

He said Idaho and Oklahoma are the only two states, aside from Illinois, that have formally indicated plans to use data other than decennial Census.

Some states have made other accommodations.

In New Jersey, voters approved a constitutional amendment during last year’s General Election to postpone the redistricting process until November.

In California, which has an August deadline, the legislature filed an emergency petition with the state Supreme Court, requesting a four-month extension to finalize new maps. The court granted the request.

Illinois lawmakers are taking a different approach. Democrats have indicated they plan to finalize their maps before June 30. Although the state Constitution sets that date for lawmakers to finish their work, Republicans have frequently noted it is not final.

Under the Constitution, if lawmakers fail to meet the June 30 deadline, a bipartisan commission would be formed to lead the redistricting process, as was done in 1981, 1991 and 2001.

Democrats have balked at the idea of letting the commission lead the process.

On Tuesday, Sen. Elgie Sims (D-Chicago) said turning over redistricting to a “group of political insiders” would “be an ultimate disservice to communities of interest.”

Williams said the different approaches to dealing with the delayed Census data is hardly surprising. “In the end, this is what federalism is. There’s no one-size-fits-all answer. The thing that may be best in one state may not be best in another,” he said.

A deep dish pizza

Overall, Williams said whatever data source states use, it is one of many components used during the redistricting process.

“States never just redistrict with the Census data. They always add more data on top,” he said, equating it to a Chicago deep dish pizza.

To meet the requirements of the federal Voting Rights Act, state leaders insert political data into a program to determine if there is racially polarized voting in certain areas.

If states are interested in factoring in communities of interest, as Illinois lawmakers have indicated, they might add economic or industry specific data into the process, Williams said. “There’s definitely layers to it,” he said.

During the initial Senate hearings, Democrats suggested that the data they use will be one of many components included in the redistricting process.

On Monday, Sen. Patrick Joyce (D-Essex) said in addition to census data, lawmakers are holding public hearings to help them understand how communities have changed in the last decade.

During a March 17 redistricting hearing, Aquino offered a similar sentiment.

Related: Questions over data, transparency, public notice arise as redistricting process begins

“The purpose of these hearings is to get the information so that we do have the best data available to come up with a map by our June 30 deadline,” he said.

Aquino’s call to use the best data is hardly new, with a similar sentiment suggested more than two decades ago.

“I think our preference would be to use the most complete data available,” Steve Brown, former House Speaker Mike Madigan’s spokesman, said in February 2000. “But this is not in Illinois yet….we’ll just buckle up and stand by.”