Here's how we came up with our predictions.

White man

Looking at the past 30-plus years of spree- and mass-shootings, this is the easiest trend to spot. Nearly two-thirds of the 67 incidents — 65.7 percent — were at the hands of a white person. Only one was committed by a female. It is indisputable: white men are most likely to commit such acts, though not exclusively.

Shooting at work

Most of the shootings in the database occurred at a place of business, either one where the shooter worked or a restaurant or other such facility. Workplace-related shootings are twice as common as school shootings, for example, which has implications for other demographic predictions. The largest category of shootings is "Other," with nearly half of all shootings taking place at some public location. Workplace shootings comprise just under a third of incidents. Of all of our predictions, this one is the most tenuous.

38 years old

By focusing on the workplace as the site of the incident, it allowed us to better refine the age of the shooter. School shootings are, somewhat obviously, undertaken by younger people. Narrowing the possible venues for an incident to workplaces and other venues, we were able to calculate the average age of men committing such crimes: 38. Include school shootings, and that age drops to 35.6.

Mental health

While mental health issues are often tricky to diagnose, it's clear that there exists a correlation between a pattern of mental illness and involvement in mass shootings. Those asked after the fact are probably more likely to have seen signs of mental illness, of course; killing a dozen people tends to taint past perceptions. The Mother Jones index has documented examples of treatment attempts, for example, lending as much objectivity as possible. And it shows that about 63 percent of those involved in mass shootings had some history of mental health issues.

Legally-purchased handgun purchased in-state

The vast majority of guns used in mass killings were obtained legally — 81.8 percent. Advocates of gun control will note that this bolsters the case for tighter restrictions, particularly given the overlap between those with mental health issues and those able to buy guns. Opponents of gun control will note that nearly a fifth of shootings used illegally obtained guns, suggesting that new controls won't prevent such shootings.

We were curious about where the guns were obtained, envisioning a possibility that those who live in states or locations with tighter gun control laws might go to another state to buy a weapon (as the shooter in the Washington, D.C., Navy Yard attack did). Almost three-quarters of the time, the guns were purchased in the same state as the incident.

As for the type of weapon, the database did most of the work on this, too. Not every shooter used a semi-automatic handgun, but at each of the workplace or "other" events, such a weapon was the most common one present.