A set of dice is intransitive (or nontransitive) if it contains three dice, A, B, and C, with the property that A rolls higher than B more than half the time, and B rolls higher than C more than half the time, but it is not true that A rolls higher than C more than half the time. In other words, a set of dice is intransitive if the binary relation – X rolls a higher number than Y more than half the time – on its elements is not transitive. More simply, A normally beats B, B normally beats C, but A does not normally beat C.
It is possible to find sets of dice with the even stronger property that, for each die in the set, there is another die that rolls a higher number than it more than half the time. This is different in that instead of only "A does not normally beat C" it is now "C normally beats A" Using such a set of dice, one can invent games which are biased in ways that people unused to intransitive dice might not expect (see Example).
The probability that A rolls a higher number than B, the probability that B rolls higher than C, and the probability that C rolls higher than A are all 5/9, so this set of dice is intransitive. In fact, it has the even stronger property that, for each die in the set, there is another die that rolls a higher number than it more than half the time.
If this game is played with a transitive set of dice, it is either fair or biased in favor of the first player, because the first player can always find a die that will not be beaten by any other dice more than half the time. If it is played with the set of dice described above, however, the game is biased in favor of the second player, because the second player can always find a die that will beat the first player's die with probability 5/9. The following tables show all possible outcomes for all three pairs of dice.
win against each other with equal probability they are not equivalent. While the first set of dice (A, B, C) has a 'highest' die, the second set of dice has a 'lowest' die. Rolling the three dice of a set and always using the highest score for evaluation will show a different winning pattern for the two sets of dice. With the first set of dice, die B will win with the highest probability (11/27) and dice A and C will each win with a probability of 8/27. With the second set of dice, die C′ will win with the lowest probability (7/27) and dice A′ and B′ will each win with a probability of 10/27.
Efron's dice are a set of four intransitive dice invented by Bradley Efron.
The four dice A, B, C, D have the following numbers on their six faces:
Each die is beaten by the previous die in the list, with a probability of 2/3:
B's value is constant; A beats it on 2/3 rolls because four of its six faces are higher.
Similarly, B beats C with a 2/3 probability because only two of C's faces are higher.
P(C>D) can be calculated by summing conditional probabilities for two events:
The four dice have unequal probabilities of beating a die chosen at random from the remaining three:
As proven above, die A beats B two-thirds of the time but beats D only one-third of the time. The probability of die A beating C is 4/9 (A must roll 4 and C must roll 2). So the likelihood of A beating any other randomly selected die is:
Similarly, die B beats C two-thirds of the time but beats A only one-third of the time. The probability of die B beating D is 1/2 (only when D rolls 1). So the likelihood of B beating any other randomly selected die is:
Die C beats D two-thirds of the time but beats B only one-third of the time. The probability of die C beating A is 5/9. So the likelihood of C beating any other randomly selected die is:
Finally, die D beats A two-thirds of the time but beats C only one-third of the time. The probability of die D beating B is 1/2 (only when D rolls 5). So the likelihood of D beating any other randomly selected die is:
Therefore, the best overall die is C with a probability of winning of 0.5185. C also rolls the highest average number in absolute terms, 3+1/3. (A's average is 2+2/3, while B's and D's are both 3.)
Note that Efron's dice have different average rolls: the average roll of A is 8/3, while B and D each average 9/3, and C averages 10/3. The intransitive property depends on which faces are larger or smaller, but does not depend on the absolute magnitude of the faces. Hence one can find variants of Efron's dice where the odds of winning are unchanged, but all the dice have the same average roll. For example,
These variant dice are useful, e.g., to introduce students to different ways of comparing random variables (and how comparing only averages may overlook essential details).
A set of four dice using all of the numbers 1 through 24 can be made to be intransitive. With adjacent pairs, one die's probability of winning is 2/3.
These dice are basically the same as Efron's dice, as each number of a series of successive numbers on a single die can all be replaced by the lowest number of the series and afterwards renumbering them.
Miwin's Dice were invented in 1975 by the physicist Michael Winkelmann.
The following intransitive dice have only a few differences compared to 1 through 6 standard dice:
Like Miwin’s set, the probability of A winning versus B (or B vs. C, C vs. A) is 17/36. The probability of a draw, however, is 4/36, so that only 15 out of 36 rolls lose. So the overall winning expectation is higher.
Warren Buffett is known to be a fan of intransitive dice. In the book a discussion between him and Edward Thorp is described. Buffett and Thorp discussed their shared interest in intransitive dice. "These are a mathematical curiosity, a type of 'trick' dice that confound most people's ideas about probability."Fortune's Formula: The Untold Story of the Scientific Betting System that Beat the Casinos and Wall Street,
Buffett once attempted to win a game of dice with Bill Gates using intransitive dice. "Buffett suggested that each of them choose one of the dice, then discard the other two. They would bet on who would roll the highest number most often. Buffett offered to let Gates pick his die first. This suggestion instantly aroused Gates's curiosity. He asked to examine the dice, after which he demanded that Buffett choose first."
In 2010, Wall Street Journal magazine quoted Sharon Osberg, Buffett's bridge partner, saying that when she first visited his office 20 years earlier, he tricked her into playing a game with intransitive dice that could not be won and "thought it was hilarious".
A number of people have introduced variations of intransitive dice where one can compete against more than one opponent.
Whatever the two opponents choose, the third player will find one of the remaining dice that beats both opponents' dice.
One can verify that, when the game is played with one set of Grime dice:
However, when the game is played with two such sets, then the first chain remains the same (with one exception discussed later) but the second chain is reversed (i.e. A beats D beats B beats E beats C beats A). Consequently, whatever dice the two opponents choose, the third player can always find one of the remaining dice that beats them both (as long as the player is then allowed to choose between the one-die option and the two-die option):
There are two major issues with this set, however. The first one is that in the two-die option of the game, the first chain should stay exactly the same in order to make the game intransitive. In practice, though, D actually beats C. The second problem is that the third player would have to be allowed to choose between the one-die option and the two-die option – which may be seen as unfair to other players.
The above issue of D defeating C arises because the dice have 6 faces rather than 5. By replacing the lowest (or highest) face of each die with "reroll" (R), all five dice will function exactly as Dr. James Grime intended:
Alternatively, these faces could be mapped to a set of pentagonal-trapezohedral (10-sided) dice, with each number appearing exactly twice, or to a set of icosahedral (20-sided) dice, with each number appearing four times. This eliminates the need for a "reroll" face.
This solution was discovered by Jon Chambers, an Australian Pre-Service Mathematics Teacher.
In analogy to the intransitive six-sided dice, there are also dodecahedra which serve as intransitive twelve-sided dice. The points on each of the dice result in the sum of 114. There are no repetitive numbers on each of the dodecahedra.
Miwin’s dodecahedra (set 1) win cyclically against each other in a ratio of 35:34.
The miwin’s dodecahedra (set 2) win cyclically against each other in a ratio of 71:67.
It is also possible to construct sets of intransitive dodecahedra such that there are no repeated numbers and all numbers are primes. Miwin’s intransitive prime-numbered dodecahedra win cyclically against each other in a ratio of 35:34.
Three or more sets of dice in each of which dice form their own intransitive cycles of superiority, and relations between the sets are intransitive too. The first example: 's meta-dice.