The first ongoing and organized research program on precognition was instituted by Joseph Banks Rhine in the 1930s at Duke University's Parapsychology Laboratory. Rhine used a method of forced-choice matching in which participants guessed the order of a deck of 25 cards, each five of which bore one of five geometrical symbols. Although his results were positive and gained some academic acceptance, his methods were later shown to be badly flawed and subsequent researchers using more rigorous procedures were unable to reproduce his results. His mathematics was sometimes flawed, the experiments were not double-blinded or even necessarily single-blinded and some of the cards to be guessed were so thin that the symbol could be seen through the backing.