Only about half of violent crimes and a third of property crimes in the US are reported to police each year. Just 46% of the reported violent crimes are cleared by police, meaning the cases ended with an arrest or a culprit was otherwise identified without the possibility of arrest– for example, the suspect is known to have died. The clearance rate for property crimes is even worse: an abysmal 18%.
Of course, clearance rates vary tremendously across the country. For instance, most homicides in San Diego are cleared, while the clearance rate for homicides in Chicago was just 20% in 2016. It's probably no coincidence that in 2016 Chicago's homicide rate was eight times higher than San Diego's (28 versus 3.5 per 100,000).
While a multiyear study covering 289 US counties concluded that “clearance rates were strongly associated with crime”, most likely through a deterrence effect, how this plays out on the ground is still being worked out. Looking at Philadelphia, it appears that homicides vary with clearance rates after a lag time. For instance, the Philadelphia Homicide Unit reported a 70% clearance rate in 2012 and the following year homicides went down 26% (331 in 2012 to 246 in 2013). However, the impact of clearance rates on homicides is not quite so obvious when we look at Chicago:
Sources: Homicide Rate: Credit: The University of Chicago Crime Lab https://www.thetrace.org/2017/01/chicago-murder-rate-fatal-shootings/
Homicide Clearance Rates: FBI; Chicago Police Department https://www.thetrace.org/2016/09/chicago-gun-violence-police-detectives/
In the above graph, homicide and clearance rates appear to vary together during the same year, which makes the deterrence hypothesis a bit problematic. It could be that in years there are fewer homicides, the police simply have more resources available to clear them. And when there are more homicides, a lack of resources makes clearance that much harder.
Next: how to improve clearance rates
Reference:
Douglas Clement (2009) Just the facts, ma’am: Crime patterns in the district vary widely, but explaining differences and trends is difficult, The Region https://www.minneapolisfed.org/publications/fedgazette/just-the-facts-maam