Key Terms
- Probability: A measure of how likely an event is, from 0 to 1
- Outcome: A single possible result of an experiment
- Event: One or more outcomes we are interested in
- Favourable outcome: An outcome that satisfies the event
- Impossible event: An event with probability 0
- Certain event: An event with probability 1
- Equally likely: Each outcome has the same probability
Must-Know Facts
- Probability is always between 0 and 1 (inclusive)
- P = 0 means impossible; P = 1 means certain; P = 0.5 means even chance
- All probabilities for a complete set of outcomes add up to 1
- P(event does NOT happen) = 1 − P(event happens)
- Fractions, decimals, and percentages can all express probability
- A standard die has 6 equally likely outcomes; a deck has 52 cards
- Always simplify probability fractions (e.g. 3/6 = 1/2)
Key Formulas
- P(event) = number of favourable outcomes ÷ total number of possible outcomes
- P(A does not happen) = 1 − P(A)
- Sum of all outcome probabilities = 1
Common Mistakes
- Probability greater than 1: Probability is always between 0 and 1 — if your answer is greater than 1, recheck
- Writing as a fraction then converting incorrectly: P = 3/5 = 0.6 = 60% — all three forms are acceptable unless specified
- Not listing all outcomes: Always count all equally likely outcomes systematically — use a list or sample space diagram
- P(not A) = 1 − P(A) only: This only works if A either happens or doesn't — check there are no other outcomes
This topic summary covers Knowledge Organiser: Basic Probability within Basic Probability for GCSE Mathematics. Revise Basic Probability in Probability for GCSE Mathematics with 13 exam-style questions and 20 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 7 of 7 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Basic Probability
Which value represents an impossible event on a probability scale?
Explain what is meant by theoretical probability and state the formula used to calculate it.
Quick recall flashcards
What is probability?
A measure of how likely an event is to occur, expressed as a number between 0 and 1
Can probability be negative?
No, probability must be between 0 and 1