The idea of using a prediction that is in code and is decoded to reveal a correct prediction of a free choice was in the British air in 1930. See Eric Impy's “Mental Miracle” in his Conjurer's Choice; and Edward Bagshawe's “The Message That Can't Be Read” in Twenty Magical Novelties, p. 37. These seem the earliest instances of using a coded prediction, these relying on making the method of decoding the message capable of producing several outcomes that match a small range of possible choices.