How do you calculate the expected value?

 How to calculate the Expected value


Last modified December 23, 2018,

A carnival has a lot of games. A standard six-sided, six-sided die can be rolled for $2. If you get a six, you'll win $10. Otherwise, you lose nothing. Are you trying to make money and want to play the game? The concept of expected value is needed to answer this question.



You can think of the expected value as the mean of random variables. This means that the expected value can be thought of as the mean of a random variable if you do a series of probability experiments and keep track of the results. The expected value represents what you should expect to occur over the course of many chances of winning a game.


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How to calculate the expected value

A discrete random variable is the example in the carnival game. This variable is not continuous. Each outcome can be taken apart from the others. To calculate the expected value of a game with outcomes x1, and x2, click here. . . . . , pn, calculate:


x1p1 + x2p2 + . . . + xnpn.


The chance of you winning nothing is 5/6 for the game shown above. Since you paid $2 to play this game, the outcome of this outcome has a value -2. A six has a 1/6 chance of appearing, and this value gives an outcome of 8. Why 8 instead of 10? We need to pay $2 to play again, and 10 + 2 = 8.


Add these probabilities and values to the expected value formula. The result is: -2 (5/6) + 8, (1/6) = 1/3. This means you can lose 33 cents per game. Yes, sometimes you may win. However, you will often lose.


Carnival Game Revisited

Now imagine that the carnival game has been slightly altered. The $2 entry fee is the same. You win $12 if you have a six-digit number. If you don't, you lose nothing. This game's expected value is -2 (5/6) +10 (1/6) = 0 While you may not lose any money in the end, you can't win any. You shouldn't expect to find a game using these numbers at your local carnival. If the carnival doesn't lose any money over the long-term, it won't be there. 

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