Rhine, along with associate Karl Zener, had developed a statistical system of testing for ESP that involved subjects guessing what symbol, out of five possible symbols, would appear when going through a special deck of cards designed for this purpose. A percentage of correct guesses (or hits) significantly above 20% was perceived as higher than chance and indicative of psychic ability. Rhine stated in his first book, "Extrasensory Perception" (1934), that after 90,000 trials, he felt ESP is "an actual and demonstrable occurrence".