A certain airport security scanner designed to detect explosives in luggage will alert the scanner' s operator whenever the piece of luggage passing under the scanner contains an explosive  The scanner will erroneously alert the operator for only one percent of the pieces of luggage that contain no explosives
Thus in ninety-nine out of a hundred alerts explosives will actually be present
ignores the possibility of human error on the part of the scanner's operator once the scanner has alerted him or her