Winning at Russian Roulette

Every time I teach stochastic processes, we discuss whether to play Russian Roulette (don’t!). In the off chance one absolutely has to play, we determine the best time to take a turn and whether it is best to spin the barrel.

I believe in teaching important life lessons in class along with operations research. But in this case, I seriously hope none of my students consider this example on Russian roulette an important life lesson. This example is good for exploring how we can quantify probabilities to confirm our intuition.

Just for fun, here are the relevant probabilities of death based on order and spinning strategy.

First, consider the odds with not spinning the barrel.

Let Ei = the event that the ith person survives (based on order of play). The first person has a 5/6 chance of survival:

If we condition on the first outcome, the second person also has a 5/6 chance of survival:

If we condition on the first two outcomes, the third person has a 5/6 chance of survival:

This makes intuitive sense. The bullet goes into one chamber where it is “preassigned” to one player of the game.

Next, consider the odds with spinning the barrel.

Let Ei = the event that the ith person survives (based on order of play). The first person has a 5/6 chance of survival:

If we condition on the first outcome, the second person also has improved chance of survival:

If we condition on the first two outcomes, the third person has an even more improved chance of survival.

Continuing in this way, we can compute the odds of death based on each player’s order.

Without spinning the barrel, someone will lose. If every players spins the barrel prior to his/her turn, there is a 33.5% chance that everyone will walk away from the game. Such a small action greatly affects the outcome of the game, especially for those who are among the last to go.

Do not spin the barrel:

Order	P(die)
1	0.1667
2	0.1667
3	0.1667
4	0.1667
5	0.1667
6	0.1667
P(someone dies)	1.0

Spin the barrel:

Order	P(die)
1	0.1667
2	0.1389
3	0.1157
4	0.0965
5	0.0904
6	0.0670
P(someone dies)	0.665

We can see here that there is no guaranteed way to win at Russian roulette. However, going last after everyone spins the barrel lowers your probability of losing by 60%.