The probability of an event with favorable outcomes will be equal to the relative frequency when the number of trials becomes very large.
You write the probability of an event as . The stands for “probability”, so is read as “the probability of ”. Here you will learn some terms that are key when talking about—and understanding—probability.
An outcome is a possible result of a trial. When you roll a die, there are six possible outcomes: 1, 2, 3, 4, 5 or 6. A sample space is the set of all the possible outcomes of a trial or an experiment. The sample space when you roll a die is therefore , which is the set of all the possible outcomes 1, 2, 3, 4, 5 and 6.
When you flip a coin, there are two different outcomes: heads or tails. The sample space when you toss a coin is therefore , which is the set of the outcomes heads and tails.
Rule
The sample space is the set of all possible outcomes of an experiment.
The sum of the probabilities of all of the outcomes equals 1.
The probability of an outcome can never be less than or greater than 1.
The probability of an outcome is less than or equal to 1.
Example 1
You roll a die. What is the sample space? What is the probability of each outcome? What is the sum of the probabilities?
The sample space is
When rolling a die, the probabilities of the different outcomes 1, 2, 3, 4, 5 or 6 are written as:
Example 2
You’re at a birthday party, and there are four bowls: The first is filled with Oreos, the second with Doritos, the third with Milk Duds and the fourth with peanuts. You close your eyes and grab a snack at random. What are the possible outcomes, and what is the sample space? What’s the probability of taking something from the bowl with Oreos, Doritos, Milk Duds or peanuts?
The possible outcomes are “bowl of Oreos”, “bowl of Doritos”, “bowl of Milk Duds” and “bowl of peanuts”.
The sample space is therefore
It’s very important that you have a good understanding of the following terms:
Theory
An outcome
is a possible result of an experiment or trial.
Sample space
is the set of all possible outcomes of an experiment or trial.
An event
is something that can occur in an experiment. One event can consist of one or more outcomes.
is a variable that represents the possible events in an experiment.
describes the possible events in an experiment and their probabilities mathematically.
Later, you will see how outcome and sample space can be described in a simple way using Venn diagrams, contingency tables and tree diagrams.