It is called the "clearance rate." The FBI keeps uniform statistics of crimes reported throughout the country, and it counts as "solved" or "cleared" all crimes for which they have developed sufficient evidence to arrest, charge and turn a suspect over to the court for prosecution.
These are "best case" numbers because the authorities may have attributed a crime to a person, but that does not necessarily mean that he did it. He still has to be tried and convicted, and even then there is room for error. Also these are reported crimes, so except in the case of murder where you have a body, there are a number of crimes in which a substantial fraction go unreported. Drug dealers, for example, are not likely to report being robbed of their drugs, and of course, the crime is not likely to be solved.
The clearance rate on murder used to be much higher, since normally people are killed by family members and close associates. But during periods of Prohibition (such as now), there are more business-related murders between people who are strangers to one another, and these drag down the solution rate. Also, a murder case remains open until it is attributed to somebody, whereas there are statutes of limitations apply to other crimes. Only 9% of arsons, for example, are ever solved.
Not counted in the murder statistics are people who die on assembly lines because the company has illegally tampered with the controls, speeding up the line to unsafe levels; people who die because of exposure to asbestos, cotton or silicon dust; people who die because they received counterfeit medications, or because they were prescribed unnecessary medical procedures so their health care providers could milk the insurance company. There is a vast amount of corporate crime that is never reported or investigated as such.
These rates vary slightly by region of the country, and there is a further breakdown by method of assault,
Here's the rules----MOST CRIMES are solved by somebody ratting on someone.. That is ex-wives, partners in crime, girlfriends, etc... The guys who WORK ALONE can usually dodge prosecution, sometimes for years.. Usually they get caught by just plain BAD LUCK..
ALSO---Felons are creatures of habit.. Whatever they're into, be it car theft, selling drugs, burglary, etc,,,, sooner or later they will do the same thing.. So the term "round up the usual suspects" from the movie Casablanca, still holds true..
In high crime areas, police are happy to arrest and convict ANYBODY, even if they did'nt actually do it.. The premise being, you must be guilty of something, so it might as well be what we arrested you for..
I.E.. If you live in a high crime area, and a guy gets shot and killed, and collapses on your front porch.. Call the cops and they'll arrest you for the crime.. Thus inner city people distrust police, and many crimes just go unreported...