Robert W. Nelson’s article “If it Ain’t Broke: The Already Reliable U.S. Nuclear Arsenal” of the April 2006 issue contained a mathematical error. The sentence should have read: “An improvement in accuracy by a factor of two, for example, decreases the required explosive yield by a factor of eight. It hardly matters, for example, if the W76 warhead detonates with a yield of 100 kilotons or 90 kilotons, when a 15-kiloton explosion will do.”