 Consequently, the New York Stock Exchange and Nasdaq exchange remained closed till September 17, 2001, the most protracted shutdown since the Great Depression.A fixed model considers the "known unknowns", but ignores the "unknown unknowns", made famous by a statement of Donald Rumsfeld.The term "unknown unknowns" appeared in a 1982 "New Yorker" article on the aerospace industry, which cites the example of metal fatigue, the cause of crashes in Comet airliners in the 1950s.