From your link, and i'm sure you left this part off just by accident:
Statistics under scrutiny: Why some experts disagree with the CRPC report on mass shooters
As eye-opening as the CRPC study was, many statisticians believe the reason the results seem so counterintuitive is that they’re incorrect. One of the more detailed analyses appeared on the
fact-checking website snopes.com and concluded that the CRPC report used “inappropriate statistical methods” which led to misleading results.
According to the snopes analysis, one of those inappropriate methods was the leaving out of the many European countries that had not experienced a single mass shooting between 2009-2015. This data would not have changed the position of the U.S. on the list, but its absence could lead a reader to believe—incorrectly—that the U.S. experienced fewer mass shooting fatalities per capita than all but a handful of countries in Europe. A more important oversight, again according to snopes, was the report's use of average deaths per capita instead of a more stable metric. Thanks to the smaller populations of most European countries, individual events in those countries had statistically oversized influence and warped the results. For example, Norway’s world-leading annual rate was due to a single devastating 2011 event, in which far-right extremist Anders Behring Breivik gunned down 69 people at a summer camp on the island of Utøya. Norway had zero mass shootings in 2009, 2010, 2012, 2013, 2014, and 2015.
An easy, though arguably insensitive, way to illustrate the shortcomings of this approach is to imagine it applied to the 9/11 attacks, which killed 2,977 people in the United States on a single day in 2001. Running that data through the CRPC formula yields the following statistic: Plane hijackings by terrorists caused an average of 297.7 deaths per year in the U.S. from 2001-2010. This is mathematically accurate, but it paints a badly distorted picture of what actually happened during those ten years.
In addition, the CRPC study went a step further and computed average annual deaths
per capita. Critics argue this further warps the data, because Norway’s population is a fraction of the U.S. population. As a result, Norway’s death rate came out more than 20 times higher than that of the U.S.—which tallied 66 deaths in 2012 alone (nearly matching Norway's total for the full study) and averaged at least one death per month for the entire seven-year data set.
A possible better alternative to the CPRC mass shooter report
The snopes analysis goes on to suggest that instead of computing the average, or
mean mass shooting deaths, a better method would be to compute the median, or
typical, number of deaths. The median is considered by many statisticians to be better at preventing individual outlier events (such as the Norway massacre) from skewing results, which leads to a more accurate day-to-day impression and country-to-country comparison. Using the CPRC’s own data and more precise per-year population data from
World Bank (the original study used only 2015 population data) to solve for the median, the snopes analysis results in a notably different list:
Typical (Median) Annual Death Rate per Million People from Mass Public Shootings (U.S., Canada, and Europe, 2009-2015):
- United States — 0.058
- Albania — 0
- Austria — 0
- Belgium — 0
- Czech Republic — 0
- Finland — 0
- France — 0
- Germany — 0
- Italy — 0
- Macedonia — 0
- Netherlands — 0
- Norway — 0
- Russia — 0
- Serbia — 0
- Slovakia — 0
- Switzerland — 0
- United Kingdom — 0
Using the median analysis, the United States is the only country examined that shows a propensity for mass shootings. The data itself supports this interpretation, as the United States endured mass shooting events all seven years, but the other countries all experienced mass shootings during only one or two years. Thus, in a typical year, most countries experience zero mass shooting deaths, while the US experiences at least a few.