Mass shootings in the US are generally categorised as events where at least four people are shot (not necessarily killed) by one or more others.
The San Bernardino, California, mass shooting (at least 14 dead) has overshadowed another that happened the same day, in Savannah, Georgia. There a gunman shot and killed a woman and injured three men.
The chart shows the numbers killed per day in 2014 and (so far) in 2015.
Picture source: www.shootingtracker.com