Mark I. Knapp, 76, a cost-research analyst at the Institute for Defense Analysis, died Sept. 26 at Montvue Nursing Home in Luray, Va., of sepsis and advanced cervical spondylosis, a neurological disorder.

Mr. Knapp worked at the then-Arlington-based institute, which is funded by the Department of Defense, for nearly 25 years before retiring in 1986.

While in the Washington area, Mr. Knapp lived in Bowie. He moved to Luray 12 years ago and had lived at Montvue for the last year.

Mr. Knapp, who was born in New York, was a pilot in the old Army Air Corps in England during World War II and in an intelligence unit during the Korean War.

He graduated from New York University with a bachelor's degree in engineering in 1951.

In the 1970s, he was a fellow with the American Association for the Advancement of Science and a member of the Southern Maryland Health Systems Agency.

His first three wives, Dorothy, Louise and Donna Knapp, died.

He is survived by his wife since 1985, Constance Saunderson Knapp of Luray; two sons from his first marriage, Michael of Columbus, Ohio, and Richard of Cleveland; a daughter from his first marriage, Elizabeth Knapp of Blacksburg, Va.; a daughter from his third marriage, Jeanine Knapp Gallagher of Cape May, N.J.; two stepsons from his fourth marriage, George Byrnes of Woodbridge and Andrew Byrnes of Reston; four stepdaughters from his fourth marriage, Lauri Chambers of Luray, Amy Epperson of Centreville, Susan Penrod of Memphis and Holly Byrnes of Berkeley, Calif.; nine grandchildren; a great-grandchild; and a brother.