By the way, Zener cards are named for Swiss psychologist Dr. Karl Zener (1903-1963) who developed the cards for J.B. Rhine's ESP tests.
We discussed how this might be done. The class has a challenge: figure out a way to duplicate the performance. If you can figure out something, you are encouraged to demonstrate it in class. Actually, several class members correctly figured out how it was done.
We did an all-class experiment with Zener cards. Prof. Cotton "transmitted" to everyone in the class. There were 5 trials. Each one wrote down what they "received" from Prof. Cotton. At the end we wrote the transmitted sequence on the blackboard and asked for a show of hands to find out how many got none right, one right, two right, and so on. Compare the class results to the calculated probabilities. Remember - these are the probabilities for random guessing. You can see that nobody got more than 4 correct, and the overall distribution fits the probabilities rather nicely. It is not impossible that someone might get 5 correct, but it would be rare.
# Right Class Theoretical 0 25/54=0.463 0.328 1 21/54=0.389 0.410 2 8/54=0.148 0.205 3 0/54=0.000 0.051 4 0/54=0.000 0.006 5 0/54=0.000 0.0003
Our result is consistent with random guessing. There is no need to invoke ESP to explain the result. We had a few more get none right than 1 right, which is slightly different from chance. Nobody got more than 2 right. Looks like no ESP here.One last practical consideration: why aren't psychics and astrologers fabulously wealthy? Anyone who can REALLY foresee the future accurately could make a killing in the stock market.
In ANY test of "psychic powers", the experiment must account for the fact that psychics can be very slippery. James Randi requires that all psychics who attempt to win his million-dollar prize agree IN ADVANCE to ALL experimental conditions and exactly what constitutes success and failure, then sign a contract including all of this. As mentioned before, in pseudo-science all failures must be explained away so no doubt will be cast on the claimed power. Failure can be blamed on "negative energy" emitted by skeptics, confusing objects in the room, time of day, or anything creative that can be invented quickly to explain a failure. See The Skeptic's Dictionary for notes on the ad hoc excuse.
James Randi offers a million dollars to anyone who can PROVE they have ESP under controlled conditions. Although some number have tried, Randi's money is still safe.
James Randi understands perfectly that the test protocol must be written to prevent all forms of trickery and ad hoc excuses for failure. You might be surprised to hear that the person best equipped to prevent trickery is a magician, not a scientist. The magician's art is that of trickery and deception, things that a scientist does not expect. In science, one is expected to be clear and honest, describing all experiments and data with great clarity and NO trickery.
James Randi once tested a dowser who claimed the ability to find metal by using his dowsing rod. A test was set up where a piece of metal was placed in one of several non-metallic containers and the lids put on. In a warm-up test, the dowser's rod correctly indicated which container had the metal, but the dowser knew in advance which container held it. In the real test, the dowser did not know which container had the metal, and he failed to locate it. An ad hoc excuse quickly followed. Randi then removed the cloth covering the test table, which was made of metal! The dowser had completely failed to detect the metal table. Test failed. For more on dowsing, see James Randi's library page. Here's another good page describing protocols for testing dowsers. Once again, The Skeptic's Dictionary has an excellent description. Here's an EXCELLENT description of a dowsing test done in Australia.