The definition they used is atleast 4, excluding the shooter.

Personally i feel 4 wounded is a good definition for a mass-shooting, if one is to include multiple fatalities it no longer shows how prevalent the shootings themselves are. It would disguise the seriousness of the issue. Including multiple fatalities specifically as well does not accurately portray the lethality.

Since the qualifier for this chart is 4 wounded or killed, it serves as an accurate representation of the amount of mass-shootings, but not the severity of them. However if one were to count only shootings with multiple fatalities it would become less accurate the more fatalities there are.

An example:

Month 1 has three shootings with two fatalities and 2 wounded each

Month 1 = 3 shootings resulting in multiple fatalities.

Month 2 has 3 shootings, 2 of them result in 5 wounded, the third results in 10 deaths and zero wounded.

Month 2 has had one shooting resulting in multiple fatalities.

If we wish to see the severity of a shooting on average, it would be better to do so with deaths/shooting.

Month one has 3 shootings, 6 dead.

6/3 = 2 = three shootings with an average of two fatalities each.

Month two has three shootings, ten dead.

10/3 = 3.3 = three shootings with an average of 3.3 fatalities each.

This might be useful to measure the effectivness of the response from both the victims and emergency services. The most effective way of minimizing the amount of fatalities from this type of data would be to compare the fatalities of any mass-shooting to the average fatalities. Shootings that greatly exceed the average can be studied to see why they differ. Location, weapon used, time of day, emergency response etc.

The chart in the article is accurate for the frequency of mass shootings, but inaccurate for their severity. Calculating only shootings with multiple fatalities give an accurate representation of severe, lethal mass-shootings, but it is innacurate regarding both the total amount of fatalities, and the total amount of shootings. But if we combine this with the total amount of shootings, we can get an accurate representation of (for lack of a better term) non-severe/severe ratio. If we assume 30 days a month, one shooting a day and one a month with multiple fatalities the ratio would be 29 shootings for every 1 shooting resulting in multiple fatalities, or 29:1.

If we were using an average it accurately tells us how many shootings we would have, along with how many deaths they result in. However it is inaccurate in telling us how widely it can fluctuate from shooting to shooting. 10 shootings, 9 of them with only wounded, and one of them with 20 dead would give as an average of 2 dead per shooting. However the one mass-shooting resulting in 20 deaths is clearly worse than the other nine.

It depends on what kind of information youre looking for.