For homicide detectives, 2020 brought good news and bad news. On the one hand, police across the nation solved more murders — in absolute numbers — than in any year since 1997, according to data reported to the FBI. On the other hand, because new homicides increased sharply, the reported rate at which killings were solved, known as the “clearance rate,” declined to a little below 50%.

The lower clearance rate in 2020 was an extension of a long, steady drop since the early 1980s, when police cleared about 70% of all homicides, and a decline that experts say was exacerbated by the pandemic. (The FBI won’t release 2021 numbers until later this year.)

In most cases, clearing a crime means at least one suspect was arrested and charged with the crime. However, individual agencies have different ways of calculating clearance, with some clearing a case once police identify a suspect, and others if an arrest is made.

At the national level, the FBI uses blunt math to calculate a clearance rate, dividing the number of crimes that were cleared — no matter which year the crime occurred — by the number of new crimes in the calendar year. By clearing old and new cases, a department’s rate in any given year could exceed 100%. This leaves the numbers prone to statistical “noise,” but they can be useful for examining trends over the long term.

Clearance has long been the primary metric that law enforcement agencies use to assess their effectiveness at solving crime. Low or declining clearance rates often lead to increased political pressure on police leadership, and calls for more hiring or funding.

For police to “clear” a crime, they usually need to identify and arrest the suspect. But according to more granular data collected by the FBI but reported by fewer agencies, at least 400 murders cleared in 2020 were solved by “exceptional means.” That means police believed they had enough evidence, but were unable to make an arrest. This occurs when the suspect has died, can’t be extradited or if prosecutors refuse to press charges. Critics say police use clearance by exceptional means — sometimes colloquially described as putting “bodies on bodies” — to artificially inflate clearance numbers.

Read the full article about the murder clearance rate by Weihua Li and Jamiles Lartey at The Marshall Project.