This number includes people shot, not just people killed. The term is "mass shooting" not "mass murder". We don't need statistics to show that we have a problem. Mass shootings are occurring more frequently and sadly the assailants are using firearms that were legally purchased. You can google "how many mass shooting in 2015" and you will get a list of websites displaying the information.
It would be helpful to have a definition of a "mass shooting". How many people need to be hit with a bullet for it to be classified as a mass shooting?
The FBI say that mass murder is four or more killed in the same event. Found this site that says 4 or more people being shot, not necessarily killed, is a mass shooting (and the shooter being killed counts toward the minimum of 4 people shot.) By their reckoning there have been 204 mass shootings so far in 2015.
http://shootingtracker.com/wiki/Mass_Shootings_in_2015
http://www.washingtonpost.com/news/...4-mass-shootings-and-204-days-in-2015-so-far/