Here’s a slightly dated (but still very significant) well-researched and factual essay on the merits of concealed carry for reducing shooting deaths by F. Paul Valone of Guns, Politics, and Freedom.
Valone cites researchers John Lott of Yale and William Landes of the University of Chicago paper studying multiple victim public shootings. Their data of records from 1976-1995 found that in states where concealed carry was adopted shootings declined by 84%, deaths reduced by 90% and injuries from shootings down 82.5%.
What is it about concealed carry that has this effect? The researchers credit the threat of armed victims as the deterrent.
“[T]he only policy factor to have a consistently significant influence on multiple victim public shootings is the passage of concealed handgun laws.”