APA Dictionary of Psychology

an entry in a table that is certain to be zero because it corresponds to an impossible outcome, as opposed to an entry that has an empirical value of zero. For example, in studying friendships, one might ask each of 15 people to indicate who among the others is a friend. This would yield a 15 × 15 contingency table or matrix, with 1 representing “is a friend” and 0 representing “is not a friend.” Although there would be empirical zeros in some cells to represent not-a-friend responses, all entries along the diagonal of the matrix are structural zeros since they represent one person’s response with regard to themself rather than to another person.