Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025
Definition
Probability is the measure of the likelihood that an event will occur, expressed as a number between 0 and 1. It quantifies uncertainty and can be calculated using various formulas depending on the context.
5 Must Know Facts For Your Next Test
The probability of an event $A$ is given by $P(A) = \frac{\text{number of favorable outcomes}}{\text{total number of outcomes}}$.
The sum of probabilities of all possible outcomes in a sample space is always 1.
If two events are mutually exclusive, the probability of either occurring is the sum of their individual probabilities: $P(A \cup B) = P(A) + P(B)$.
For independent events, the probability of both occurring is the product of their individual probabilities: $P(A \cap B) = P(A) \times P(B)$.
Conditional probability, denoted as $P(A|B)$, represents the probability of event $A$ occurring given that event $B$ has occurred.