Zener cards

Zener cards are cards used to conduct experiments for extra-sensory perception (ESP), most often clairvoyance. Perceptual psychologist Karl Zener designed the cards in the early 1930s for experiments conducted with his colleague, parapsychologist J. B. Rhine.

Design
There are five different Zener cards: a hollow circle (one curve), a Greek cross (two lines), three vertical wavy lines (or "waves"), a hollow square (four lines), and a hollow five-pointed star. There are 25 cards in a pack, five of each design.

When Zener cards were first used, they were made out of a fairly thin translucent white paper. Several subjects or groups of subjects scored very highly until it was discovered that they had often been able to see the symbols through the backs of the cards. A redesign made it impossible to see the designs through the cards under any conditions. A subsequent deck featured an illustration of a building at Duke University on its reverse side, but the use of a non-symmetric reverse design allowed the deck to be exploited as a one-way deck.

Use in experiments
In a test for clairvoyance, the person conducting the test (the experimenter) picks up a card in a shuffled pack, observes the symbol on the card, and records the answer of the person being tested for ESP (the subject), who must correctly determine which of the five designs is on the card in question. The experimenter continues until all the cards in the pack have been tested.

Third parties may oversee or videotape an experiment to make sure it is conducted fairly. While it is especially important to ensure that the subject cannot see any cards and does not receive any vocal or visual cues from the experimenter, other methods of cheating are possible. To this end, physical separators may be placed between the experimenter and the subject. As with other ESP tests, experiments with Zener cards have used elaborate methods to keep the subject from seeing the cards or the experimenter, sometimes placing the subject in a separate room.

If the subject is informed during the test that specific guesses were correct or incorrect, card counting can increase their accuracy; also, poor shuffling methods can make the order of cards in the deck easier to predict. In his experiments, Rhine first shuffled the cards by hand but later decided to use a machine for shuffling.

Although Zener cards are usually used to test for clairvoyance, they may also be used to test for telepathy, in which case one subject will draw a card and attempt to mentally project the image on it to the mind of another subject. Here, the statistical tendency of the receiver to report a specific design must be taken into account &mdash; for example, they might tend to report receiving an image of a square more than other images &mdash; so the deck used must contain an equal number of cards of each design.

Statistics
Since the five symbols are equally distributed throughout the deck, guesses by a non-psychic in a controlled test would on average result in 1 / 5 &times; 25 = 5 correct guesses per deck and a success rate of 20%. The more times a person is tested on a pack of 25 cards, the more statistically meaningful his or her score becomes; it's not at all unusual for a person to have as many as 8 or as few as 2 correct guesses on a single trial (yielding scores of 32% and 8%, respectively), but if the subject is not psychic the scores for several runs are expected to average out to 20% over time.