Police commissioner: Last year our city experienced a 15 percent decrease in the rate of violent crime  At the beginning of that year a new mandatory sentencing law was enacted, which requires that all violent criminals serve time in prison
Since no other major policy changes were made last year, the drop in the crime rate must have been due to the new mandatory sentencing law
At the beginning of last year, the police department's definition of violent crime was broadened to include 2 crimes not previously classified as violent. 