Twenty-five cards, and the number five
Five symbols: circle, cross, wavy lines, square, star. Five of each, twenty-five in a deck. You call the card before it is turned. Chance says you will get five right.
This is the protocol J. B. Rhine ran at Duke University in the 1930s, and it is a large part of why parapsychology exists as a laboratory discipline rather than a parlour one. Rhine’s early runs produced scores well above five and made him famous. The subsequent decades produced the more useful result: when the deck is properly shuffled, and when the caller cannot see the card, the back of the card, the experimenter’s face, or the experimenter’s hands, the score comes back to five.
Protocol P-02 runs the series honestly. The deck is shuffled cryptographically, the card is committed before you call, and your score is reported against chance with no thumb anywhere near the scale.
The Survey has run this since 1974. Chance is still five. We keep running it because the day it stops being five is a day worth having a record of.