Most American police officers have traditionally been required or encouraged to carry their weapons while off duty and to actively intervene in criminal incidents at all times. Police unions often base a portion of their salary arguments on the fact their their members, unlike other public official union members, must carry out their duties at all times. Police administrators often feel that additional officers would be necessary to replace the deterrence factor provided by armed off-duty police. However, little quantitative data exist on the cost-benefit ratio of arming off-duty police officers. A survey of New York City Police Department adjudications of cases in which officers discharged their weapons while off duty found that 55.8 percent of the cases involved self defense, 2.5 percent accidents, and 9.0 percent were miscellaneous cases. While 75 percent of the cases were related to law enforcement or order maintenance, the remaining 25 percent involved accidents which caused death or injury, criminal acts, and similar situations. Officers off duty usually have little time to plan and consider actions when faced with a crime while off duty, while officers on duty can usually make plans enroute to the crime site. Off-duty officers also usually are alone or with family or friends rather than other officers, and are generally not in uniform. They are also much more likely to be under the influence of alcohol. Police officers often feel that they may be executed unless they resist any armed criminals at any time. However, most criminals are deterred from shooting police officers by the comprehensive investigation which follows such shootings. Further research on these issues is recommended. Footnotes which include references are provided.