First, the chart:
Per the FBI, law enforcement agencies can clear, or “close,” offenses by arrest or by exceptional means, such as the death of an identified offender. Most cases are cleared by arrest.
Some of the crimes cleared in one year may have been committed in prior years.
There is no hard-and-fast threshold for an acceptable clearance rate. That said, Oakland’s rate is abysmal. No wonder Oakland’s the most dangerous city in the US!
Crimes are easier to clear in smaller towns and cities, where the perps are likely to be local and known by the police.
Crimes are harder to clear when eyewitnesses are uncooperative, whether due to “no-snitch” attitude or fear of reprisal.
Simply hiring more police may not have much impact on clearance rates. Per Variability of crime clearance among police agencies (Scott et al):
“More important than a department’s budget or number of officers may be how the department structures its criminal investigations, the extent of communication between investigative units and patrol, the relationship between investigative units and district attorneys, and other more difficult-to-measure qualities. A great deal more research is needed to discern why agencies vary in the ability to clear crime.”
Reference:
Scott, Thomas L., Charles Wellford, Cynthia Lum, and Heather Vovak. "Variability of crime clearance among police agencies." Police Quarterly 22, no. 1 (2019): 82-111. https://doi.org/10.1177/1098611118796597
Links:
https://www.safewise.com/safest-cities-america/#list
https://www.statista.com/statistics/217685/most-dangerous-cities-in-north-america-by-crime-rate/
https://ucr.fbi.gov/crime-in-the-u.s/2019/crime-in-the-u.s.-2019/topic-pages/violent-crime
https://cde.ucr.cjis.gov/LATEST/webapp/#/pages/explorer/crime/crime-trend
https://ucr.fbi.gov/crime-in-the-u.s/2017/crime-in-the-u.s.-2017/topic-pages/clearances