A couple of years ago, Mother Jones did a study of mass shootings which attempted to characterize these awful events. Some of their conclusions were robust — such as the finding that most mass shooters acquire their guns legally. However, their big finding — that mass shootings are on the rise — was highly suspect.
Recently, they doubled down on this, proclaiming that Harvard researchers have confirmed their analysis1. The researchers use an interval analysis to look at the time differences between mass shootings and claim that the recent run of short intervals proves that the mass shootings have tripled since 2011.2
Fundamentally, there’s nothing wrong with the article. But practically, there is: they have applied a sophisticated technique to suspect data. This technique does not remove the problems of the original dataset. If anything, it exacerbates them.
As I noted before, the principle problem with Mother Jones’ claim that mass shootings were increasing was the database. It had a small number of incidents and was based on media reports, not by taking a complete data set and paring it down to a consistent sample. Incidents were left out or included based on arbitrary criteria. As a result, there may be mass shootings missing from the data, especially in the pre-internet era. This would bias the results.
And that’s why the interval analysis is problematic. Interval analysis itself is useful. I’ve used it myself on variable stars. But there is one fundamental requirement: you have to have consistent data and you have to account for potential gaps in the data.
Let’s say, for example, that I use interval analysis on my car-manufacturing company to see if we’re slowing down in our production of cars. That’s a good way of figuring out any problems. But I have to account for the days when the plant is closed and no cars are being made. Another example: let’s say I’m measuring the intervals between brightness peaks of a variable star. It will work well … if I account for those times when the telescope isn’t pointed at the star.
Their interval analysis assumes that the data are complete. But I find that suspect given the way the data were collected and the huge gaps and massive dispersion of the early intervals. The early data are all over the place, with gaps as long as 500-800 days. Are we to believe that between 1984 and 1987, a time when violent crime was surging, that there was only one mass shooting? The more recent data are far more consistent with no gap greater than 200 days (and note how the data get really consistent when Mother Jones began tracking these events as they happened, rather than relying on archived media reports).
Note that they also compare this to the average of 172 days. This is the basis of their claim that the rate of mass shootings has “tripled”. But the distribution of gaps is very skewed with a long tail of long intervals. The median gap is 94 days. Using the median would reduce their slew of 14 straight below-average points to 11 below-median points. It would also mean that mass shootings have increased by only 50%. Since 1999, the median is 60 days (and the average 130). Using that would reduce their slew of 14 straight short intervals to four and mean that mass shootings have been basically flat.
The analysis I did two years ago was very simplistic — I looked at victims per year. That approach has its flaws but it has one big strength — it is less likely to be fooled by gaps in the data. Huge awful shootings dominate the number of victims and those are unlikely to have been missed in Mother Jones’ sample.
Here is what you should do if you want to do this study properly. Start with a uniform database of shootings such as those provided by law enforcement agencies. Then go through the incidents, one by one, to see which ones meet your criteria.
In Jesse Walker’s response to Mother Jones, in which he graciously quotes me at length, he notes that a study like this has been done:
The best alternative measurement that I’m aware of comes from Grant Duwe, a criminologist at the Minnesota Department of Corrections. His definition of mass public shootings does not make the various one-time exceptions and other jerry-riggings that Siegel criticizes in the Mother Jones list; he simply keeps track of mass shootings that took place in public and were not a byproduct of some other crime, such as a robbery. And rather than beginning with a search of news accounts, with all the gaps and distortions that entails, he starts with the FBI’s Supplementary Homicide Reports to find out when and where mass killings happened, then looks for news reports to fill in the details. According to Duwe, the annual number of mass public shootings declined from 1999 to 2011, spiked in 2012, then regressed to the mean.
(Walker’s article is one of those “you really should read the whole thing” things.)
This doesn’t really change anything I said two year ago. In 2012, we had an awful spate of mass shootings. But you can’t draw the kind of conclusions Mother Jones wants to from rare and awful incidents. And it really doesn’t matter what analysis technique you use.
2. This is less alarming than it sounds. Even if we take their analysis at face value, we’re talking about six incidents a year instead of two for a total of about 30 extra deaths or about 0.2% of this country’s murder victims or about the same number of people that are crushed to death by their furniture. We’re also talking about two years of data and a dozen total incidents.↩