According to Koch's essay in 1981,

According to Koch's essay in 1981, there are 91 police officers killed while in duty and the cases that were solved showed that seven percent of the convicts had a history of homicide. Also in Koch's essay in between 1976 and 1977 in the Big Apple, 85 persons arrested for homicide had a previous arrest for murder, six had two previous arrests for murder and one had four previous murder arrests. So how long do we hold these convicts in prison? Based on twentieth century fund survey the average time the prisoner served in the United States for a first degree murder is only ten years, and the lower degree murder charge means an even shorter sentence time in many case and states.