Zener Cards - Design

Design

Originally, tests for ESP were conducted using a standard deck of playing cards. However, there are many confounding variables involved with this methodology. When this methodology is used, a participant is only credited a correct prediction for guessing both the number and suit of the card. This means that the chance of correctly guessing a card is greatly reduced, and there is a lot of ambiguity involved with statistical analysis. Another problem with using playing cards is that many people will have a preference for a particular card, number or suit (Irwin 1999) and will constantly suggest that as their prediction for the next card that will appear. This led to the development of a set of cards known as Zener cards, invented by Karl Zener.

There are just 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.