Watch The Red, Not The Black

Abstract

On a recent business trip to New York City, I heard the red, not the black, watch the red, not the black,... being chanted repeatedly from the vicinity of Seventh Avenue and Forty-fourth Street. On casual investigation I found a street hustler rapidly shuffling three common playing cards?two of black suits, one of red?on a blanket covering a cardboard box. At the end of the shuffling, which was accompanied by the chant, spectators were offered the opportunity to select the red card from among the three cards facedown on the blanket. Not surprisingly, bets were required, usually in multiples of $20, be fore a spectator was allowed to choose. Also on occasion, a second innocent specta tor was offered the same opportunity after the first one had failed. After the thirty sec onds or so required for the entire process, the card shuffling and chanting began again. Examination of this game, and some possible variations, offers a study in ap plied probability. Since students of virtu ally any discipline are now being required to gain a speaking acquaintance with the fundamental ideas of probability and ex pected value,1 this game offers an answer to the question, But when will I ever use this material in the real world? Seventh Ave nue and Forty-fourth Street is as real a world as th?se students are ever likely to see.