What Drives the Jail Population


Jail admissions are down, the average daily jail population is down, but length of stay in jail is up. Why?

Felony arrests are down, felony case filings are down; but daily felony court caseloads, the average felony court case length, including time to pleas of guilty are all up.

What’s going on?

Notes on data sources for this page can be found by scrolling through each chart.


There are two main types of admissions to the jurisdiction of the Cook County Sheriff’s Office:

  1. Confinement in jail
  2. Supervision in the community (for example, electronic monitoring)

The total number of admissions to jurisdiction of the Sherriff’s Office, both confinement and community supervision, has dropped by almost half between 2008 and 2017.

Census of Jail Facilities, Annual Survey jails (weekly admissions / 7 X 365).


The average daily population confined in jail has also declined (down 24%), although not as steeply as annual admissions (down 49%).

The total average daily population, including people in jail and those in electronic monitoring programs, has only dropped 9%, while the electronic monitoring (EM) program has grown substantially by 234%. The biggest jump took place after 2011, when the Sheriff’s Office expressly expanded the EM program.

Compared to those confined in jail, the percentage of people under the jurisdiction of the Sheriff’s Office who are in the electronic monitoring program has risen substantially—from 6% to 29%, while the confined population share of the total has dropped from 94% to 71%.

Cook County Sheriff’s Office, 2016.


Meanwhile, the average length of time spent in jail by the shrinking jail population has become longer.

Another way to understand this is to note that In 2000, it took 10 admissions to jail a year to fill one jail bed for a year. By 2017, it only took 6.7 admissions to fill the same bed.

There are two ways this can happen:

  1. There are fewer people facing low level cases entering the jail, resulting in a smaller jail population overall but a higher proportion of people facing serious cases. These cases take longer to adjudicate, thus raising the overall average time spent in jail (Compositional change).
  2. The same kind of cases that took less time to prosecute in the past now take longer (Transactional change).

We do not have a direct measure of length of stay, but instead use the formula: average length of stay = average daily population / daily admissions (from AOIC).


One reason total jail admissions are down is because the police are making fewer felony arrests. Not only has the overall volume of felony arrests gone down but the composition of charges faced by those arrestees has changed. Lower level felony arrests (class 3 or 4) have fallen more steeply (by 27%) than higher level felony arrests (class 1 or 2), which have fallen by 20%.

This gives some support to explaining the rise in the average length of stay in jail as a result of the diversion of less serious cases, such that the remaining jail population is on average composed of more serious cases.

Daily arrest data for Cook County is an estimate, derived from annual arrest data retrieved from Illinois Criminal Justice Information Authority’s Criminal Justice Data Profile portal and dividing the provided figures by 365 in normal years and 366 in leap years (Illinois Criminal Justice Information Authority 2017).


Fewer felony arrests have led to a decline in felony case filings by the State’s Attorney’s Office (SAO). Felony filings have fallen by 29%, ten points less than felony arrests have fallen (39%); while misdemeanor filings have dropped even more sharply, by 46%.

The shallower drop in felony cases filings by SAO than in felony arrests by police, demonstrates the filtering role that SAO plays in determining which cases go to court, and adds further weight to the explanation that a change in the composition of the court bound population is resulting in an increase in the average length of stay of people in jail.

The Administrative Office of the Illinois Court’s “Annual Report of the Illinois Courts” provides counts of the number of felony and misdemeanor cases filed or reinstated in the Cook County Circuit Court every year. We turn this into a daily rate for consistency with the rest of our analyses.


Between 2005 and 2014, the number of felony cases faced by the courts on one day has decreased, increased, and started to decrease again; resulting in an overall drop of 11%.

Were this overall trend to continue, it would give credence to the explanation that part of the increase in average length of stay in the jail is due to an increase in the time it’s taking in court to resolve some cases of the same type today than in the past.

The Administrative Office of the Illinois Court’s “Annual Report of the Illinois Courts” provides the number of cases pending at the end of the year, which we interpret here as the daily caseload for December 31.


Between 2002 and 2015, the average time it takes in court to resolve felony cases has increased 51%. If this substantial increase is due entirely or primarily to a reduction in the number of less serious cases filed with the Court, than theoretically, in the long run, the Court should begin to reduce the backlog of cases that it inherits from one year to the next.

These figures are calculated from the Cook County State’s Attorney case-level data obtained through FOIA by the Chicago Data Cooperative.


The time it takes for half of all cases to be resolved through a plea of guilty has increased over the last six years by approximately 33%. For cases initiated in 2012, it took approximately six months for half to be resolved as guilty pleas. For cases initiated in 2016 it took about eight months.

Over that time, the mix of cases reaching a plea of guilty does not appear to have changed substantially, giving further credence to the explanation that at least in part, the time some types of cases have been treated in the court has changed.

These figures are calculated from the Cook County State’s Attorney case-level data obtained through FOIA by the Chicago Data Cooperative.


Fewer people are going to jail, but they’re staying longer. Why?

The jail population has changed
As people convicted of lesser crimes are diverted from extended court adjudication and pre-trial detention, the jails hold a higher percentage of people who have been charged with more serious crimes.

The way cases are handled has changed
It may also be taking longer to adjudicate some types of cases as a result of shifting practices in court (among judges, prosecutors, and defense attorneys).

Can existing reform strategies continue to reduce the overuse of jail?

Whatever the mix of these two causes, the net consequence of recent trends is a hardening jail population at current reduced levels, driven more by length of stay than by volume of admissions. With a jail population increasingly resistant to further reductions, existing strategies will witness diminishing returns without either:

More widespread supervised release from custody of pre-­trial felony defendants,


Reduced case processing times for felony cases, especially for those headed to prison.

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