Ok so after having an interest in these things for a while I decided to produce them. I've been using them at the start of each Paranormal rip to get people in the mood and break the ice. Now I'm selling them on my site HERE.
The Cards: 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.
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.
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 — for example, they might tend to report receiving an image of a square more than other images — 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 result in about 1 / 5 of 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.