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Probability Basics

Middle School

Definition

Probability is a number between 0 and 1 representing the likelihood of an event occurring. 0 means impossible, 1 means certain.

P(A) = (favorable outcomes)/(total outcomes)

Classical probability definition

0 ≤ P(A) ≤ 1

Probability range

P(A^c) = 1 - P(A)

Complement probability

Example 1

Find the probability of rolling 3 or less on a die.

Example 2

Find P(at least one head) when flipping a coin twice.

Discovered by: Pascal and Fermat (1654)

Correspondence about gambling problems led to the foundations of probability theory.

Risk assessment

Weather forecasting

Diagnostic test accuracy