It rarely makes sense to draw big conclusions or make public policy on the basis of anecdotes. But the plural of "anecdote" is data, and sometimes one-off events are useful in crystallizing lessons to guide policymakers and inform the public.
So it was with the Pittsburgh-area rampage this week in which a teenager bearing two kitchen knives is accused of injuring 21 high school classmates and a security guard -- but none of them were killed. It's hard to imagine an anecdote that better illustrates what decades of data show: that for purposes of life and death, the weapon matters.
It's called "the instrumentality effect," and we owe the original scholarly findings (more than four decades old at this point) to the eminent University of California criminologist Franklin Zimring. Others, including one of us (Cook), have validated and built upon his insights.