I came away from Katrina and the builders with a kind of theory: under conditions of complexity, not only are checklists a help, they are required for success. There must always be room for judgment, but judgment aided—and even enhanced—by procedure.

He also did something curious: he designed a little metal tent stenciled with the phrase Cleared for Takeoff and arranged for it to be placed in the surgical instrument kits. The metal tent was six inches long, just long enough to cover a scalpel, and the nurses were asked to set it over the scalpel when laying out the instruments before a case. Th