Fewer crimes usually mean higher clearance rates

Occasionally I receive suggestions for this newsletter from readers and well-wishers. Most of them don’t work as potential topics – usually because the general idea is wrong, the data isn’t available for writing, or the idea doesn’t fit the general theme of my newsletter and writing.

However, every now and then I come up with a great idea that is worth exploring and can be meaningfully studied using existing data.

That’s what happened when Oren Gur, policy advisor and research director of the Philadelphia District Attorney’s Transparency Analytics (DATA) Lab, proposed a theory about clearance rates. Gur texted me one evening many months ago and said, “I think if the homicide rate is high for several years and then declines, based on the clearance rate calculation, the clearance rate will be very high in subsequent years.” high. Because they had a lot of open cases from previous years, but the denominator is now low.”

It immediately seemed like a potentially interesting topic.

To take a step back, clearance rates are a surrogate measure used by police departments and the FBI to assess the proportion of crimes that end in an arrest. The FBI defined Clearance rates like:

A law enforcement agency reports that a crime is solved by arrest or solved for crime reporting purposes when three specified conditions are met. The three conditions are that at least one person:

Arrested.

Charged with committing the crime.

Handed over to the court for prosecution (whether following an arrest, a court summons, or a police notification).

Crimes can also be resolved on an exceptional basis, meaning that law enforcement has identified the perpetrator, has enough evidence to make an arrest, knows exactly where the perpetrator is, but does not arrest him for reasons beyond the authority’s control can. In the case of a murder-suicide, for example, the case is exceptionally resolved because the perpetrator has died.

There is another component to consider about clearance rates that makes them a potentially misleading statistic. The denominator in the equation is the number of crimes reported in a given year (let’s say there were 78 murders in a city in 2024), while the numerator is the number of crimes reported in that year through arrest or exemption were clarified, regardless of when the crime was committed. A theft on December 31, 2023 that is solved the next day is a 2023 felony and a 2024 release.

This sometimes leads to strange results, such as Costa Mesa (CA) reporting a 1,700 percent murder clearance rate in 2020 (2 murders, 34 solved). Or Slidell, LA, which technically reports a -400 percent clearance rate in 2014 (-1 murder, 4 solved).

Given the strangeness of calculating clearance rates, let’s hypothetically oversimplify it and say an agency has the resources to investigate about 50 percent of one year’s murders, 10 percent of the previous year’s murders, and a handful of murders older than one year year, which we call a constant of 5 old murders per year.

Now let’s take this hypothetical city and apply what has essentially happened in the US in terms of rising and falling murders since 2020. Murders rose sharply in 2020 and have fallen steadily since then, to about the same level as 2019.

In this hypothesis, the agency’s murder clearance rate would fall from 70 percent to 60 percent in the year of the increase, before rising steadily as the murder rate fell until it reached 74 percent a few years later. In this scenario, the clearance rate has increased, but the agency has not gotten any better at solving this year’s crimes. The authorities simply have to solve more older murders in the numerator than the number of murders decreases in the denominator.

This hypothesis seems consistent with what we see in Philadelphia, where murders are at their lowest level in a decade and police solve 70 percent of murders until October.

And also Baltimore, where murder is commonplace 24 percent while BPD reports a big jump in the city’s murder clearance rate.

The effects of old murders on clearance rates can be seen in NYPD data for 2023 and 2024, where an identical 45 percent of the current year’s murders were solved by September of each year, but a larger number of older murders were solved last year than this year, resulting in a slightly higher overall clearance rate last year.

The real world isn’t quite as simple as the hypothetical world, but there is evidence that cities with declining murder rates tend to see rising murder clearance rates. To show this, I collected data from cities that had at least 20 murders in a given year and reported both murder and clearance data for that year and the following year between 2000 and 2023.

Cities that experienced a decrease in murder cases tended to experience increasing clearance rates, whereas cities that experienced an increase in murder cases tended to experience decreasing clearance rates over this period. Nearly 60 percent of cities that saw a year-over-year decline in murders between 2000 and 2023 also saw an increase in murder clearance rates. In most cities that saw an increase in murders during this period, clearance rates fell each year.

The overall context isn’t particularly strong for any individual agency – there are plenty of examples of places with declining murder rates and declining clearance rates, like New York above. While fewer murders do not necessarily mean a higher clearance rate, it does make sense that rising or falling crime rates nationally are a large factor in the overall trend of the nation’s clearance rate. But other factors – such as an agency’s use of resources – can undoubtedly play a role.

With this in mind, it is not particularly surprising that clearance rates for almost every type of crime increased in 2023. The only exception to the increasing clearance rates was motor vehicle theft, which is also the only crime type to see an increase in reported crimes nationwide.

Clearance rates are the formal official statistic used to measure a police department’s effectiveness in solving cases. However, it is not particularly good analytically because the formula is formulated in such a way that it is potentially misleading.

We built one dashboard for the New Orleans City Council, which measures the arrest rate of homicides and shootings in New Orleans, which I believe provides a better measure of the department’s effectiveness. The arrest rate is the number of crimes resulting in an arrest in a given year divided by the number of crimes in that year.

It’s not perfect because it doesn’t take into account this murder-suicide and doesn’t recognize the work done this year to solve last year’s case, but it’s much better as an indicator of effectiveness over time. And the news for New Orleans is very positive, as arrests accounted for more than 53 percent of fatal shootings this year, the highest since data collection began in 2011 (though there’s obviously still a lot of room for improvement!).

Clearance rates are a statistic that can be useful, but can also be manipulated – intentionally or accidentally. An apparent increase in effectiveness can be explained just as easily as a change in workload. This does not mean that clearance rates are useless, but conclusions about what a change in clearance rates means should be viewed critically given the inherent flaws in the underlying data.

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