U.S. Army Sgt. Joseph Chamberlain and Spc. Alex Egan, both from 1st Squadron, 2nd Stryker Cavalry Regiment, provide security during a patrol in Baghdad, Iraq, Nov. 29, 2007. (U.S. Army photo by Spc. Jeffery Sandstrum)
A Brown University study came up with some figures (my comments in italics):
H/t to Political Animal for pointing to the study.