A little math can be a dangerous thing: Washington Post on Crime rates

It is one of the least-told stories in American crime-fighting. New York, the safest big city in the nation, achieved its now-legendary 70-percent drop in homicides even as it locked up fewer and fewer of its citizens during the past decade. The number of prisoners in the city has dropped from 21,449 in 1993 to 14,129 this past week. That runs counter to the national trend, in which prison admissions have jumped 72 percent during that time. . . .

If you want to drive down crime, the experience of New York shows that it's ridiculous to spend your first dollar building more prison cells," said Michael Jacobson, who served as New York's correction commissioner for former mayor Rudolph W. Giuliani (R) and now is president of the Vera Institute of Justice, which studies crime-fighting trends worldwide. . . .

Perhaps as intriguing is the experience in states where officials spent billions of dollars to build prisons. From 1992 to 2002, Idaho's prison population grew by 174 percent. the largest percentage increase in the nation. Yet violent crime in that state rose by 14 percent. . . .

There appears to be a simple explanation for why both prison population and crime can fall in New York. When murders fall by 70 percent, can you really keep on expanding the prison population? Note that the prison population has fallen by a third, but violent crime in the city has fallen by much more than that.

It would have been helpful if they had put the numbers in per capita rates, rather than comparing numbers 10 years apart. For example, I guarantee you that Idaho's population grew by more than 14 percent, though less than 174 percent. Thus it would appear the crime rate did fall as the prison population grew.