Originally, tests for ESP were conducted using a standard deck of playing cards. However, there are many variables involved with this methodology. When this methodology is used, a participant is only credited with a correct prediction for guessing both the number and suit of the card. That means that the chance of correctly guessing a card becomes a lot lower; there then can be ambiguity involved with the 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, a system of designs 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.