It is widely held that the average U.S. serviceman was nineteen years old, as evidenced by the casual reference in a pop song ("19" by Paul Hardcastle); the figure is cited by Lt. Col. Dave Grossman ret. of the Killology Research Group in his 1995 book "" (p. 265). However, it is disputed by the Vietnam Helicopter Flight Crew Network Website, which claims the average age of MOS 11B personnel was 22. This compares with 26 years of age for those who participated in World War II. Soldiers served a one-year tour of duty. The average age of the U.S. military men who died in Vietnam was 22.8 years old.