The findings from two separate reports released in the last month—one by the FBI and the other by progressive investigative media outlet Mother Jones—have been offered up as evidence that "mass shootings" are occurring more frequently than ever. The truth, however, is a little more complicated.

To be fair, the authors of the FBI's "active shooter" report explicitly cautioned their study was not about mass shootings, although this caveat was later ignored in much of the news media's coverage of it. But a recent article in Mother Jones asserted that we have "entered a new period in which mass shootings are occurring more frequently." This report relied on the well-known list compiled by Mother Jones, which has been one of the key sources used to support the claim made in the last few years that the incidences of mass shootings have accelerated.

There are some problems with the Mother Jones list, however, when comparing it with data I've collected on mass killings. In my research on mass murder, which has resulted in four peer-reviewed academic publications and a book, I've identified more than 1,200 mass killings that have occurred in the U.S. since 1900. Of these cases, 161 were mass public shootings, which I've defined as incidents that occur in the absence of other criminal activity (e.g., robberies, drug deals, gang "turf wars," et cetera) in which a gun was used to kill four or more victims at a public location within a 24-hour period. Aside from a few minor differences, this definition is largely similar to the one ostensibly used by Mother Jones.

The first problem with the Mother Jones list, which contains 67 cases, is that it significantly underreports the incidents occurring between 1982 and 2013 that meet its definitional requirements. The data I've collected show there were 114 mass public shootings, which means the Mother Jones list missed more than 40 percent of the cases that took place in the U.S. during this 32-year period. (My analyses of the Mother Jones data include all 67 cases. However, four of these cases—the 2013 incidents involving Pedro Vargas and Dennis Clark, the 2007 shooting by Tyler Peterson, and the 2006 case involving Kyle Huff—were not included in my dataset because they were, in my judgment, "mass shootings" that mostly took place in residential settings.)

The second problem is that the underreporting in the Mother Jones list grows more severe the further back in time we go. When looking at the data by decade, we see that the Mother Jones list captured 89 percent of the 18 cases from 2010-2013, 69 percent of the 29 cases from 2000-2009, 55 percent of the 42 cases from 1990-1999, and just 32 percent of the 25 cases from 1982-1989. As I show below, the combination of these two problems has made it possible for those relying on this list to conclude that mass shootings are growing more and more frequent over time.

Why does the Mother Jones list suffer from an underreporting problem that gets worse for the earlier cases? Due to a lack of detailed information about the search methodology used to compile the list, it's difficult to say with complete certainty. The most likely reason is that the Mother Jones list relied exclusively on news reports as a source of data, and news coverage tends to be less accessible for the older cases.

In my research on mass murder, I've used the FBI's Supplementary Homicide Reports (SHR) data to anchor the search for more detailed news reports on mass killings. The SHR data has its flaws, but it is the most comprehensive homicide dataset available that reveals, among other things, when and where most mass killings (including mass public shootings) have taken place in the United States.

Mass public shootings have actually been more common than what the Mother Jones list indicates. The two datasets also yield some different conclusions regarding trends in the prevalence of mass public shootings since 1982. Whenever we look at crime trends over time, it's necessary to adjust for population growth. Because mass public shootings are, fortunately, a rare event, I've calculated the annual rate per 100 million of the U.S. population (as opposed to the rate per 100,000 that's commonly used to measure crime trends) across the 1982-2013 period for both datasets.

As we see in the above chart, which displays annual rates based on the Mother Jones list, the data suggest the mass public shooting rate has generally increased since 2006. Indeed, seven of the 11 highest annual rates since 1982 have been within the last eight years, which would support the claim about the ever-growing incidences of mass public shootings.

But when we look at annual rates derived from the more complete dataset above, we see that only two of the 11 highest rates have occurred within the last eight years. Instead, six of the 11 highest rates were observed during the eight-year period between 1988 and 1995.

The claims about the recent increase in mass public shootings are valid as long as we restrict our focus to the 1996-2013 period. As rates of crime and violence began to fall in the latter half of the '90s, so, too, did those for mass public shootings. Compared to the 1996-2006 period, there has been an uptick since 2007.

But within the broader context of the 32-year period, the annual rates since 2007 have been mostly typical. The average rate per 100 million over the 1982-2013 period was 1.31. Since 2006, two years (2008 and 2012) had above-average rates, four years (2007 and 2009-2011) had rates that hovered around the average, and two years (2006 and 2013) had below-average rates. Given that the average annual rate from 2007-2013 (1.51) is nearly the same as it was for the 1982-1995 period (1.50), we've recently returned to incidence levels commonly observed during the 1980s and early 1990s.

Reducing the occurrence of mass public shootings is implicit to the debate over whether they have recently increased. Yet asking why mass public shootings have recently increased, even if only modestly since the mid-1990s, is the wrong question. Rather, what truly needs explaining is why the mass public shooting rate during the 1996-2006 period (an annual average of 1.00) was lower than at any time during the last 30 (or even 40) years.

The late-1990s and early-2000s coincided with a bustling economy, the ban on assault weapons, a rising prison population, increases in the number of police, a fading crack epidemic, and the aging of the baby boomers beyond their peak crime years. It's currently unknown whether these factors (or any others) were responsible for the decline in mass public shootings. Still, determining why the mass public shooting rate dropped, which is much easier said than done, may shed light on whether it's possible to curb this type of violence in the future.