# Mathematical expectation

The mathematical expectation of any bet is defined as follows: the sum of all possible gains and losses multiplied by their relative probabilities. Written as a formula, we have:

 e= (w × p) + (-v × 1)

e = mathematical expectation
w = gain on the winning bet
p = probability of the win
v = value of the loss
l = probability of the loss

Let's apply that to a few of examples: a simple coin-toss game, an "unbalanced" coin-toss, the single number roulette bet mentioned on the house edge page, the even-money outside bets under European roulette rules and the "perfect pairs" blackjack side bet.

## Coin toss 1

The payoff for a win is even money, or 1 to 1, and the probability is 50%, or 1/2, for both win and loss - giving us the following formula:

 (1 × ½) + (-1 × ½) = 0

The mathematical expectation is 0. Neither side has an advantage.

## Coin toss 2

In this game, you bet one dollar, pay your opponent a dollar when you lose, but he pays you only ninety cents (\$0.90) when you win. That leads to the following formula:

 (0.9 × ½) + (-1 × ½) = - 0.05

The mathematical expectation is - 0.05, or -5%.

## Single number roulette bets

Winning bets are paid at 35 to 1, probability of a win is 1/37 (your number out of the total of 37 numbers) and probability of a loss therefore 36/37. That leads us to this formula:

 (35 × (1/37)) + (-1 × (36/37)) = - 0.027

The mathematical expectation is - 0.027, or 2.7%.

Here are two wagers with multi-tiered outcomes:

## "Even-money" European roulette bets

Winning bets are paid at even money and the probability is 18/37. However, if the ball falls in the "zero" compartment, half the bet is returned, and the probability of zero is 1/37. This gives us the following calculation:

 (1 × (18/37)) + (-1 × (18/37)) + (-½ × (1/37)) = - 0.0135

The mathematical expectation is - 0.0135, or 1.35%.

## "Perfect pairs" blackjack sidebet

In this bet, the payoff is received if a pair is dealt. The size of the win depends on the kind of pair card received - mixed colour (AH + AC/AS), matching colours (10H + 10D) or "perfect" (6C + 6C). The payoffs and their probabilities are listed in the table below, based on a six-deck game:

HandPayoffProbability
Mixed pair5 to 112 / 311
Coloured pair10 to 16 / 311
Perfect pair30 to 15 / 311
No pair-1288 / 311

This leads to the following calculation:

 (5 × (12/311)) + (10 × (6/311)) + (30 × (5/311)) + (-1 × (288/311)) = - 0.05787

The expectation is -0.05787, or a house edge of 5.5787%.

As you can see, the method of calculation for the more complex bets is the same as the simple ones.

Based on a total of \$1000 wagering:

Coin toss 1: 1000 × 0 = 0. Expected loss: \$0.

Coin toss 2: 1000 × 0.05 = 50. Expected loss: \$50.

Roulette inside bets: 1000 × 0.027 = 27. Expected loss: \$27.

Roulette "even money" bets: 1000 × 0.0135 = 13.5. Expected loss: \$13.5.

Blackjack "perfect pairs" side bet: 1000 × 0.05787. Expected loss: \$57.87.

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