Standard Normal Tables

Introduction

The steps below will tell your probability of losing less than a given number of units in a gambling session. Conversely you can also determine the probability of winning more than a given number of units or falling within any given range of units. This will work given that you bet the same amount every time and that you are gambling on any event that has only two possible outcomes. For example betting on any column of 12 numbers in roulette is permissible since you either lose one unit or win two units. Blackjack may not be used because there are several possible outcomes depending on whether you double, split, or get a blackjack.

The following are variable definitions used:

• p = Probability of winning
• w = Payoff for winning per unit bet. This does not include the original wager. For example, w on an even money bet would be 1.
• n = Number of bets made
• x = Maximum number of units won in question.

Next, follow the following steps:

1. Calculate p*w + p - 1 (this is the expected return per trial)
2. Multiply n by line 1 (this is the expected return for the session)
3. Calculate p*w² + (1-p)
4. Subtract line 1 squared from line 3 (this is the variance for one trial)
5. Multiply line 4 by n (this is the variance for the session)
6. Take the square root of line 5 (this is the standard deviation of the entire session)
7. Add 0.5 to x
8. Subtract line 2 from line 7
9. Define Z as line 8 divided by line 6
10. Look up the probability corresponding to the value of Z (for example if Z=2.15 then the probability is 0.9842)

Confused? There is an example after the table.

Example

Let’s suppose you want to know the probability of winning more than 5 units in a session of 50 one column bets (which pay 2:1) in roulette. This is 1 minus the probability of winning 5 or less. Below are the values of the following steps.

These are the constants:

• p = Probability of winning = 12/38 = .3158
• w = Payoff for winning per unit bet = 2
• n = Number of bets made = 50
• x = Maximum number of units won in question = 5

These are the values of the steps:

1. Calculate p*w + p - 1 = ((12/38)*2 + 12/38 - 1 = -.0526
2. Multiply n by line 1 (-.0526 * 50 = -2.6316)
3. Calculate p*w² + (1-p) (.3158*4 + 1-.3158 = 1.9474)
4. Subtract line 1 squared from line 3 (1.9474 - .0028 = 1.9446)
5. Multiply line 4 by n (1.9446 * 50 = 97.2317)
6. Take the square root of line 5 (97.2317^1/2 = 9.8606)
7. Add 0.5 to x (0.5+5 = 5.5)
8. Subtract line 2 from line 7 (5.5-(-2.6316) = 8.1316)
9. Define Z as line 8 divided by line 6 (8.1316/9.8606 = 0.8247 )
10. Look up the corresponding probability in the table below. (The probability value for Z=.82 is 0.7939)

Thus the probability of winning 5 or less is 79.39% and the probability of winning more than 5 is 20.61%.

Written by: Michael Shackleford