S.L.A. Marshall made up his figures from whole cloth. There's no evidence he ever did the research he claimed to have done upon which those figures are based.

Below quote from http://www.warchronicle.com/us/combat_historians_wwii/marsha.... Emphasis not in original This calculation assumes, however, that of all the questions Marshall might ask the soldiers of a rifle company during his interviews, he would unfailingly want to know who had fired his weapon and who had not. Such a question, posed interview after interview, would have signalled that Marshall was on a particular line of inquiry, and that regardless of the other information Marshall might discover, he was devoted to investigating this facet of combat performance. John Westover, usually in attendance during Marshall's sessions with the troops, does not recall Marshall's ever asking this question. Nor does Westover recall Marshall ever talking about ratios of weapons usage in their many private conversations. Marshall's own personal correspondence leaves no hint that he was ever collecting statistics. His surviving field notebooks show no signs of statistical compilations that would have been necessary to deduce a ratio as precise as Marshall reported later in Men Against Fire. The "systematic collection of data" that made Marshall's ratio of fire so authoritative appears to have been an invention.