A quick recap of probability
Probability is a number from 0 to 1, found by reasoning or by trials.
Stage 9 probability builds on what you already know, so a quick refresher first.
Every probability is a number on a scale from 0 to 1. A probability of 0 means impossible, 1 means certain, and means an even chance.
For equally likely outcomes:
Theoretical probability is worked out by reasoning — a fair dice gives without rolling. Experimental probability is found by doing trials and counting: successes total trials.
This year the focus is combined events — two or more events happening together or one after another — and a powerful new tool, the tree diagram. That is what the rest of this guide covers.
- Probability runs from 0 (impossible) to 1 (certain).
- For equally likely outcomes, P = favourable ÷ total.
- Theoretical probability is reasoned; experimental comes from trials.
- This year focuses on combined events and tree diagrams.