In a contingency table, the joint events are mutually exclusive because they consist of unique combinations of individual events that are mutually exclusive. For an individual to be described by one of the joint events, the individual must be described using each of the two attributes measured in the table. Since the possibilities for each of the two attribute are mutually exclusive, there can be only one way for an individual to be labeled with each pair of attributes. Therefore there can only be one joint event in the table that describes the individual.