Earlier quoted context omitted.
This article has a graph of mass shootings, offenders and victims from 1980 to 2010. http://boston.com/community/blogs/crime_punishment/2012/08/n... > Based on data extracted from official police reports to the FBI, the figure below shows annual incident, offender and victim tallies for gun homicides in which at least four people were murdered. Over the thirty-year time frame, an average of about 20 mass murders have…
Interesting, this page from the Washington Post says that of the worst 11 shootings, 5 of them happened since 2007, and that doesn't even include Sandy Hook. http://www.washingtonpost.com/blogs/wonkblog/wp/2012/12/14/n...
It doesn't list Starkweather and Fugate's 10 person killing spree in 1957, or James Edward Pough's 1990 rampage in Jacksonville that killed 11 and injured 6, or Mark Barton's 1999 rampage that killed 12 and injured 13, or Leung Ying's 1928 California spree that killed 11 injured 4, or Nidal Hassan's 2009 Fort Hood spree killing 13, injuring 30, or Patrick Sherill's 1986 Oklahoma spree killing 14 injuring 6, or Ronald Simmons 1987 Arkansas rampage killing 16 injuring 4, or George Banks 1982 Pennsylvania killing of 13 injuring 1.
Since the article leaves out so many well known cases it can not be considered reliable regarding its comprehensiveness. The other article which actually did complete counts of all known cases should be considered far more authoritative.