Bertrand's chord, Buffon's needle, and the concept of randomness

Abstract

Two old problems in probability theory involving the concept of randomness are considered. Data obtained with one of them—Bertrand's chord problem—demonstrate the equivocality of this term in the absence of a definition or explication of assumptions underlying its use. They also support two propositions about probabilistic thinking: (1) upon obtaining an answer to a question of probability, people tend to see it as the answer, overlooking tacit assumptions on which it may be based, and tend not to consider the possibility of other assumptions from which different answers would follow; (2) perceptual balance—moderate uniformity of density of distribution—is a determinant of the degree to which a two-dimensional pattern is perceived as random.