Casualties (killed, wounded) from mass shootings are not continuously distributed; this suggests an alternative approach — given the high variance (shown in Figure 1 below for a subsample of the data) — I estimate a negative binomial regression (quasi-maximum likelihood).
Figure 1: Mass shooting fatalities (dark red), and wounded (light red). Orange denotes Trump administration. Source: Mother Jones accessed 8/5/2019, author’s calculations.
Estimate over 1982M08-2019M08 (through 8/5/2019 for August):
casualtiest = -2.60 – 0.64 bant + 1.54 trumpt + 0.014 time
Adj-R2 = 0.048, SER = 29.95, NOBS = 445. Bold denotes significance at 10% msl, standard errors corrected for overdispersion (GLM option in EViews).
In words, each year under a Trump administration,
18 154 percent more people are injured or killed in mass shooting events than would otherwise occur. As discussed in this post, a year during the Trump administration is associated with 8.64 over 5 more events/year than would otherwise occur. [correction to interpretation 8/7 8:15am Pacific; h/t Rick Stryker]