With gun shootings, you immediately get into some definitional issues. Depending on how one defines a “mass public shooting,” the answers might turn out to be different. There is also surprisingly little historical data about mass murder in the United States to go back all the way to the nation’s founding.

Grant Duwe , director of research and evaluation at the Minnesota Department of Corrections, assembled a data set going back 100 years for a 2007 book titled, “ Mass Murder in the United States: A History .” He used the FBI Supplementary Homicide Reports, which date from 1976, and then supplemented the FBI reports with news reports (principally The New York Times) dating from 1900.

Duwe says the Times turned out to be a relatively reliable guide for mass murders across the country, since much of the post-1976 information also turned up in the contemporaneous FBI reports. As far as he knows, he is the only person who has assembled such a historical data set.

According to his research, he has identified 156 mass public shootings in the United States in the past 100 years.

Duwe defines a mass public shooting as an incident in which four or more victims are killed publicly with guns within 24 hours — in the workplace, schools, restaurants and other public places — excluding shootings in connection with crimes such as robbery, drugs or gangs.  (Note that this would exclude a number of “mass murders” that sometimes get lumped into the data, such as the Beltway sniper who killed 10 people over a three-week period in 2002.)

Since 2005, when the assault ban expired, there have been 32 such mass public shootings, including seven in 2012, Duwe said. So that’s just over 20 percent of all mass public shootings, which is much less than Clinton’s 50 percent.

Here’s a breakdown per decade of Duwe’s data. It is important to note that these are raw figures; the United States had far fewer people 50 or 100 years ago.

Mass Public Shootings per Decade

1900s : zero

1910s: 2

1920s: 2

1930s: 9

1940s: 8

1950s: 1

1960s: 6

1970s: 13

1980s : 32

1990s: 42

2000s: 28

2010s (three years) : 14

Duwe says that 2012 was certainly a horrific year, but it is too early to tell if it signals an ominous trend. The worst year for public shootings was in 1991, when eight incidents took place, he said. With seven incidents, 2012 ranks second, along with 1999 — when the assault ban was in effect. (Duwe says his research shows that assault weapons are used in a relatively small number of cases, but they do result in far more wounded victims)

We ran this data past a spokesman for Clinton, but he declined to comment or offer an explanation for where the former president got his facts. That always makes us suspicious.

Still, as our colleague Brad Plumer has  noted on Wonkblog, a different definition of mass shooting might yield a different picture. Mother Jones magazine  tabulated its own data over the past three decades and came up with these figures:

Shootings before, during and after assault ban

1982-1994: 19

1995-2004: 16

2005—2012: 27

This gets us a little closer to Clinton’s claim — it suggests that more than 40 percent of mass shootings in the past 30 years have taken place since the assault weapon ban ended. But that’s still not the same thing as he said — “half of all of them in the history of the country.”

Moreover, the reasons behind a spike or decline in gun violence  are often much more complex than a single factor, such as the expiration of a law. In general, overall crime rates, including homicides, have declined in recent years because of a variety of factors.

The Pinocchio Test

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 show 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.