Want to dazzle pals with your maths skills and snag free drinks in the process? All you need is a set of dice.
Let’s play a dice game. I have three dice (shown in the above image). They are similar to regular six-sided dice except they have slightly different numbers on them. Each die is different. I’ll label them Red, Green and Blue so that we can differentiate them. The meshes for the three dice are shown here.
We’re going to choose and roll one of the dice. I’ll even let you choose first. You are free to select any die you like (Red, Green or Blue), then I’ll pick one of the two remaining dice. We then enter into a battle: we both roll our dice and the highest-scoring number wins a point. We’ll roll 20 times and declare a winner as the one with the most points. Simple?
It might not be a surprise to my suspicious readers, but I’m going to predict that I’m probably going to win the battle. In fact I’ll go as far to say that I think I’ll win the battle over 98 per cent of the time! (Even if in the event of a tie at 10 points each I concede the win to you). OK, you say, I somehow forced you into picking one of the dice? No problem, let’s do it again.
You can chose to keep the same die or select another one (you can even have the die I just rolled, if you like!) I predict I’m going to win again. We can keep going all day. It does not matter which of the dice you choose, I’ll select one of the other dice and win over 98 times in every 100 games. Huh, what’s going on? That does not make sense! What’s the trick?
There’s no trick, it’s just that you’re thinking in a transitive way (don’t worry I’ll explain what that means shortly).
How to Play
First, I’m going to explain the strategy of how I play. Then I’ll explain why it works:
If you select the Red die, I’ll select the Blue one (and probably win).
If you select the Green die, I’ll select the Red one (and probably win).
If you select the Blue die, I’ll select the Green one (and probably win).
Yes, it’s that simple!
You might want to go away and make a set of these dice for yourself to use as you read this article (and then use them to scam your unwitting friends out of free drinks by betting against them).
So what’s special about the dice? Well, nothing really. They are striped with numbers, but why this “trick” works is that the relationship “is more likely to roll a higher number” is intransitive.
What does that mean? A transitive function or property is something in which all comparisons are performed according to the same dependent criteria. For example, if Albert is older than Bob, and Bob is older than Charlie, then we know that Albert must be older than Charlie too. This is because “is older than” is a Transitive function. However, if I told you that Albert is friends with Bob, and Bob is friends with Charlie, you can’t assume that Albert is also friends with Charlie! Friendship is Intransitive.
For our dice set, the function “is more likely to roll a higher number” is intransitive.
Need more convincing about the concept? At first glance this may appear counterintuitive, so let’s look at the classic game of Rock-Paper-Scissors, which is also an intransitive game.
Rock smashes Scissors, and Scissors cut Paper, but Paper also wraps Rock.
If you are playing Rock-Paper-Scissors and you know which symbol your opponent was going to select, you could always select the device that was its nemesis and be sure of a victory.
Get it? OK, let’s go back to the dice…
Our dice are constructed in a similar cycle such that the value “is more likely to roll a higher number” sequences around in a loop.
Red is more likely than Green
Green is more likely than Blue
Blue is more likely than Red
Because I asked you to select your die first, I can always select the colour next along the cycle!
Now that we have the background out of the way, let’s look at some numbers. By it’s nature, rolling dice is random and we can exploit that with our function “is more likely to be higher”.
Below are the meshes for the dice we are using in this example:
If you look closely, you will see that each dice has just three distinct numbers (repeated twice).
The Red one: 2,4,9
The Green one: 1,8,6
The Blue one: 3,5,7
Rolling the red die, there are three equally likely outcomes (each with one-third probability). The same can be said for the green die.
Multiplying these together, we see that there are nine possible (equally likely) outcomes for these pair of dice. In five of these combinations the red die wins. Thus, probablity of the red die winning against the green die is 5/9. This is shown below as a tree diagram:
A similar diagram can be constructed for all possible combinations of rolles for the green and blue dice. Again the probability is weighted, and again the advantage is 5/9.
Finally, with the blue and red dice we can again complete the cycle with a 5/9 probability.
By selecting the correct die against one freely selected by our opponent we can gain always gain a measurable advantage. The 5/9 advantage is small, but by repeating the experiment a few times (in our example by repeating the roll 20 times), we can increase our odds considerably.
Now go out and have fun with maths!
Intransitive Dice [DataGenetics]
Nick Berry is the president of DataGenetics. He is an active member of the privacy community and speaks at various events about the legal and ethical aspects of data collection, use and destruction.