What are we measuring?
A crime can be considered solved, or "cleared", by law enforcement agencies in two basic ways--by arrest or by exceptional means. When a crime is solved by arrest, three conditions must be met: at least one person has been arrested, charged with the offense and turned over to the courts for prosecution.  According to the FBI's Uniform Crime Reporting (UCR) Program, the calculation for the clearance rate is based on the number of crimes, not the number of persons arrested.  In order to meet the requirements for clearing a crime based on exceptional means, the law enforcement agency must have identified the offender and the offender's location, gathered enough evidence to make a charge and turn the offender over for prosecution, and encountered some circumstance that prevented the offender from being arrested, charged and prosecuted.  Examples of those circumstances include the death of the offender or the refusal of the victim to cooperate with the prosecution. 
The data used for this measure came from a web tool provided by National Public Radio (NPR) based on data compiled from the FBI. Data were available for only ten of the fourteen police departments in the county, but those ten represent over 90 percent of the county's population.  The Fulton County Sheriff and university police departments were excluded because those agencies are generally involved in very few arrests or criminal investigations.  The percentage of crimes solved is calculated by dividing the number of crimes solved in the year by the number of new crimes in the same year.  Since some crimes solved may have occurred in a previous year, the clearance rate in some cases may be over 100 percent.
Part 1 crimes include murder and non-negligent homicide, rape, robbery, aggravated assault, burglary, larceny-theft, motor vehicle theft and arson.  
Why are we measuring it?
The clearance rate is one measure of the effectiveness of law enforcement agencies in combating crime.  It is also important to society's sense of justice and belief that those who commit serious crimes do not go unpunished.  In reality, crimes are often very difficult to solve and police agencies must decide how to balance their resources between efforts to solve crimes and efforts to prevent them.  Clearance rates are an important measure but should never be used as the only measure of the effectiveness of law enforcement agencies.
How are we doing?
The combined clearance rate for the ten police departments for which data was available declined slightly from 18 percent in 2011 to 15.7 percent in 2015.  The decline was due mainly to a drop in the clearance rate for violent crimes, which fail from 45.1 percent in 2011 to 34.7 percent in 2015.  The clearance rate for property crimes remained relatively steady with a 2015 rate of 12.4 percent.  In comparison, the FBI reports the average Part 1 crime clearance rate for metropolitan U.S. counties in 2015 to be 23.1 percent, 51.5 percent for violent crimes and 18.8 percent for property crimes.