How Does Expected Value Work?

Theory

The Expected Value

The expected value is, in the long run, the same as the mean,. It is calculated in this way:

μ = E(X) = i=1mx i P(X = xi)

Example 1

You throw a die. What is the expected value of the number you roll?

Make a table with the possible outcomes and their probabilities:








xi 1 2 3 4 5 6







P (X = xi) 1 6 1 6 1 6 1 6 1 6 1 6







μ = E(X) = 1 1 6 + 2 1 6 + 3 1 6 + 4 1 6 + 5 1 6 + 6 1 6 = 3.5

μ = E(X) = 1 1 6 + 2 1 6 + 3 1 6 + 4 1 6 + 5 1 6 + 6 1 6 = 3.5

The expected value is E(x) = 3.5.

Example 2

You insure your laptop with an insurance company. The policy of the insurance company is to only compensate you under two circumstances:

  • Your laptop is stolen.

  • Your laptop is broken.

In the first case you’ll be compensated $400, while in the second case you’ll be compensated $200.

Assume the probability of your laptop being stolen is 2%, and the probability of your laptop breaking is 4%. To keep it simple, let’s assume that these probabilities are the same any given year.

1.
Let X be the compensation you receive from the insurance company in a year. Find the expected value of X.
2.
What is the least premium you would have to pay in order for the insurance company to not lose money on this insurance policy?

1.
From the assignment you have
P (X = laptop stolen) = 0.02

and

P (X = laptop broken) = 0.04

The expected compensation during any given year then becomes

E(X) = 400 0.02 + 200 0.04 = $16

E(X) = 400 0.02 + 200 0.04 = 16.

2.
As the expected compensation any given year is $16, the premium has to be at least $16 a year for the insurance company to not lose money.

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