Not as hard as some might think. Are manslaughters counted in the murder rate, does it matter if it's voluntary or involuntary? Do we put justifiable homicides in the murder rate? What if we can just charge it as aggravated assault. What about when there's no charge, do we write it out as a murder or a suspicious death until we have a suspect? Accidental shooting? Potentially self inflicted?

I would not be surprised if the murder rate reduction is significantly attributable to changes in statistical reporting. I know a few cities that play games with the numbers even on murders.