“Half of all mass killings in the United States have occurred since the assault weapons ban expired in 2005, half of all of them in the history of the country.”

— Former President Bill Clinton, at the Consumer Electronics Show in Las Vegas, Jan. 9, 2013

A colleague spotted this eye-popping statistic by the former president and wondered if it was correct....

 In the highly charged debate over guns, it is important for politicians on both sides to get their facts straight. In this case, the available data shows that Clinton was way off-base in his assertion, making an exaggerated claim — which his office would not even defend.

 Ordinarily, this might have been a Four Pinocchio claim. Given the fuzziness of the data and questions about definitions, we are going to cut Clinton a bit of slack in the final ruling. But such uncertainty in the data means politicians need to be very careful in making claims about gun violence.