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Zener cards are specialized cards used in parapsychology/extra-sensory perception research. They are named after Karl Zener, the psychologists who invented them in the 1930s. They were used most extensively by J. B. Rhine but continue to be synonymous with the world of psychic research and parapsychology today.
Unlike common playing cards, Zener cards have only five figures (a circle, a cross, three wavy lines, a square and a five-pointed star). This is basically due to the fact that in a standard deck of 52 playing cards, the odds of guessing the correct card is just too low, even though using playing cards would be a far more robust test of actual, factual and reliable ESP ability. The new cards were developed to reduce the amount of information that would be needed to correctly identify a card, hence the basic but distinct shapes. After switching to the Zener cards, results were staggering in favour of showing ESP was a common and real ability... until they realised the cards were printed on thin paper and the patterns (helpfully simplified compared to the standard 52 playing cards) could be seen from the reverse side.
In the standard test, the experimenter randomly (or at least supposedly randomly) takes a card, keeps it hidden, and then the subject tries to
guess "remotely view" the card. After a large number of tries, a number of correct guesses above the one expected by random chance would indicate psychic ability. Or more likely, a conscious or subconscious leak of information between the examiner and the subject. Or even more likely, they were just that person who happened to get a statistical fluke. This is a common problem in psychic research where people assume that any deviation from the mathematical average must be evidence of psychic powers.
There were also other problems with the actual random distribution of the Zener cards. Being presented in a standard deck of 25 cards with 5 of each pattern, the distribution wasn't truly random and certain combinations could be ruled out easily enough. For instance there could be no more than 5 of any particular card, whereas with a truly random selection the odds of a particular shape being drawn would always be 1 in 5. As the deck was so limited in size, card counting would also be possible and indeed quite easy compared to trying to count the cards of a poker game. With these factors combined, Zener and Rhine's insistence that the odds of a correct guess of an individual card due to chance (as opposed to ESP) was just 1 in 5 is actually an underestimate.
The basic layout of the test can be seen at the beginning of Ghostbusters, with the added twist that wrong answers are punished with electric shocks, and Peter Venkman (Bill Murray) is faking the results as an excuse to shock a college dweeb and flirt with a girl. Presumably, paranormal investigators aren't this dishonest in reality. Presumably.
 See also
- Zener ESP cards, entry at The Skeptic's Dictionary
- The mascot of the PodBlack Cat skeptical blog is a catgirl playing with Zener cards.
- Deviations: A Skeptical Investigation of Edgar Cayce’s Association for Research and Enlightenment, an account of Michael Shermer's visit to the A.R.E., their use of the Zener cards and their misuse of statistics.