The Deck, the Run, and the Arithmetic

The whole design rests on one number. A Zener deck holds twenty-five cards, five each of circle, cross, wavy lines, square and star, so a subject calling all twenty-five in order will average five hits by chance. Everything the Duke laboratory published is a statement about how far a subject's score departed from five per run, expressed in standard deviations and accumulated across many runs. That is a different kind of claim from anything psychical research had made before.

The Society for Psychical Research had spent half a century arguing about the credibility of a witness to a single striking event, a dispute with no natural end, because each side could always question the other's judgement about a story. Rhine replaced the story with a tally. He also separated conditions that had previously been run together, testing clairvoyance by having nobody look at the target, telepathy by having an agent concentrate on it, and precognition by asking for the order the deck would take after a later shuffle, and treating these as three claims needing three designs. Whether anything real was measured is contested. That the question became one about procedure and arithmetic rather than about the honesty of a witness is not.