The San Bernardino shooting, which killed 14 people and wounded 17 others, came less than a week after a man killed three people and wounded nine in a shooting rampage at a Planned Parenthood clinic in Colorado Springs, Colorado. In October, a gunman killed nine people at a college in Oregon, and in June a white gunman killed nine black churchgoers in South Carolina.
But the frequency of the recent large-scale gun attacks isn't an outlier. According to a crowdsourced database of mass shootings (defined here as an event where four or more people are wounded or killed by bullets), since 2013 there has been only one seven-day period without any mass shootings — April 8 to 15, 2015.
Still, the number of mass shootings to date this year has already exceeded the totals to date for each of the previous two years, and the San Bernardino shooting — which has had the largest number of fatalities in a mass shooting since Sandy Hook in December 2012 — brings the number of mass shootings this year to 353.