Gun Violence in the USA: calendar trends

It's nearing the end of 2015, and the US has observed over 330 337 mass shooting incidents, about 1 per day. Although the Paris attacks remind us the United States does not have a monopoly on gun violence, americans still dominate the developed world in long-run statistics. They rank, as a nation, somewhere between levels seen in Mexico and Paraguay.

Comparing firearm-related deaths, including accidental, by country. Original Article.

Unites States gun violence is not homogeneous. Within the US, we know there are certain hot spots mostly along the tiled northwest - southeast axis.

From from Business Insider

Gun violence is also a subset of all forms of violence. Statistics for total murders in countries are relatively easy to come by, but shooting-specific cases, especially when no-one is killed (but several wounded), have been less rigorously gathered. For instance, that statistic of 330 shootings comes courtesy of the user-compiled shootingtracker.com, which define a "mass shooting" as

when four or more people are shot in an event, or related series of events, likely without a cooling off period.

Since 2013 contributors have compiled incidents of those injured and killed by guns, linking with confirmed news reports. Here is a geographic distribution of Shooting Tracker data(along with density information) :
A surprising number of perpetrators are labelled "unknown" (about 60% of total), implying the assailant got away. Naturally enough, by knowing the dates of these shootings we also know which day they occurred. Although their data is too recent to show annual trends (which is well-known to be generally decreasing), it turns out day-of-week information can be surprisingly useful on another level.

Let me take on step back. In parallel to browsing shootingtracker, I came across a Washington Post article that highlighted 2015 gun violence by calendar day:

Gun violence in America for 2015: January to December.

Though visceral, the calendar graph doesn't advance the discussion beyond a reaction of "wow, that's a lot of mass shootings". But it's a start. Going down columns of dates, it appears there are an abundance of "3+ victim" red-colored days on weekends. Meanwhile June seems to have more victims than, say, January. Coincidence? I asked myself how much could seasons and specific weekdays influence murderers? The above calendar gives hints to the non-random qualities of seemingly random gun violence.

Returning then to the Shooting Tracker website, I arranged their January 2013 - Nov 2015 data into Weekday and Monthly statistics. Having the number of wounded is especially useful since wounded outnumber killed by a ratio of 3:1, which can help magnify otherwise hidden trends. And it is quite fair to include injured in the tally given the psychological impacts of being shot. And, by definition, the wounded are the only survivors of a mass shooting event.

Once compiled, it's quick to re-arrange the data. Below are the monthly values, normalized by victims per day (so that February, for instance, can be compared with July):

The jump in victims is strikingly clear during the summer season. From a low of 3 victims per day in the US in January, the killed and injured nearly double in the summer (more so the injured). An uptick in December is also apparent. Why is it that injure account for more of the trend than killed? Perhaps summer months contain more bystanders in public areas. Conversely winter months have fewer people outdoors in colder areas. But I can't see how this explains seasonal violence in warm climate states such as Alabama and Louisiana.

Whether temperature plays a factor or not, with the May-September and December violence in mind, there is a clearly a connection with holiday periods and more victims. That's even more obvious in the day-of-week chart below. Averaging Sunday to Saturday values (number of killed or wounded per week day, divided by 52), we see the weekend is the real killer.

This bar graph clearly shows how weekend shootings are more than double the weekday values. Despite media attention drawn to school shootings, it appears in-class periods have relatively fewer shootings on a monthly and day-of-week basis. Unlike the the monthly values, there is an uptick both in total killed as well as wounded during weekends.

Finally, here are the shooting statistics broken into weekly segments. Though noisy, the overall seasonal trend for the number wounded is apparent:

With all these results in mind, what might be the cause(s)? Are shooters motivated to create violence on days they know more victims are available to be shot? Is it simply pure probability that more bystanders get in the way of bullets (say, in a busy shopping mall)? Are shooters themselves more restless on off-days? (of course, that would presuppose they had a regular Monday-Friday employment schedule).

Or perhaps Shooting Tracker contributors are collecting more of their data on weekends than weekdays (likely they have jobs that extend beyond this free website). Let us crunch a few numbers. In 2013 there were 10.5 shooting deaths per 100,000 Americans. Firearm homicides are one third this value, at 3.5 per 100k. Meanwhile Shooting Tracker logged 503 deaths in 2013, which accounts for only 0.16 of the 3.5 homicides, or about 5%. This is unsurprising, as mass shootings are known to contribute marginally to all gun-related fatalities (thank goodness). Since the percentage is low, there is room for measurement error, so we cannot rule out the possibility that news reports are more thoroughly inspected on weekends. Yet I also find it hard to imagine this could explain a 250% weekend bias, and would not account for any retroactive weekend data collection.

Given the uncertainties, I have no deep thesis to present beyond presenting the questions posed by these two bar charts. Neither can I likely claim to be the first to discover this, (but have yet to find much research on these trends). I hope that the added details of such user-driven statistics will advance our understanding of violence beyond the notion that "Americans Love Guns". They certainly do love guns, but there are additional influences at work. Some week days and some months create more violence than others. As to which months these are, they are clearly non-random. What could weather, holidays, or weekends have to do with mass shootings? I have no idea for certain, but clearly something is going on.