Unit E: Statistics and Probability

Chapter 2: Probability

Probability compares the number of favourable outcomes to the total number of outcomes.
Probability can be stated as a fraction, decimal, percent, ratio, or statement.
The formula for probability is

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Odds are written as a ratio or statement, whereas probability can be written as a decimal, fraction, ratio, percent, or statement.
Odds compare desirable to undesirable events or vice versa, whereas probability compares a desired event to total possible outcomes.
Odds can be expressed using any two positive numbers, whereas probability ranges from 0 to 1 (or 0% to 100%).

Odds in favour compare the number of favourable outcomes to the number of unfavourable outcomes.
Odds are always stated as a ratio or statement.
The formula for odds in favour is

odds in favour = number of favourable outcomes : number of unfavourable outcomes

The odds against an event are the odds of an event not occurring.
The formula for odds against is

odds against = number of unfavourable outcomes : number of favourable outcomes

To find the probability when the odds in favour or odds against are given, it is useful to complete the following table.