Probability of Single Events — Year 8 Revision Notes
These notes cover the probability scale, calculating the probability of a single event, and using the fact that all probabilities add up to 1 — all at Year 8 (Stage 8) level.
The probability scale
Every probability lies between 0 and 1. A probability of 0 means an event is impossible, 1 means it is certain, and 0.5 means it is as likely as not. Probabilities can be written as fractions, decimals or percentages — so a half can be ½, 0.5 or 50%.
Key Facts & Formulas
- 0 ≤ P(event) ≤ 1
- P = ½ = 0.5 = 50%
Tips
- A probability is never negative or above 1.
- Be ready to switch between fractions, decimals and percentages.
Calculating probability
When outcomes are equally likely, the probability of an event is the number of favourable outcomes divided by the total number of outcomes. Rolling a 4 on a fair dice has 1 favourable outcome out of 6, so P(4) = 1/6. Always simplify the fraction if you can.
Key Facts & Formulas
- P(event) = favourable outcomes ÷ total outcomes
- P(4 on a dice) = 1/6
Tips
- Count the total number of equally likely outcomes carefully.
- Simplify the probability fraction where possible.
Total probability and the complement
The probabilities of all possible outcomes of an event add up to 1. This means the probability of an event not happening is 1 minus the probability that it does. If P(winning) = 0.2, then P(not winning) = 1 − 0.2 = 0.8.
Key Facts & Formulas
- Sum of all probabilities = 1
- P(not A) = 1 − P(A)
Tips
- Use 'all probabilities add to 1' to find a missing value.
- For 'not happening', subtract from 1.
Revision Checklist
- I can place events on the 0 to 1 probability scale
- I can calculate the probability of a single event
- I can write a probability as a fraction, decimal or percentage
- I can find the probability of an event not happening
Frequently Asked Questions
Do all the probabilities have to add up to 1?
Yes. For a single event, the probabilities of every possible outcome always add up to 1. This is very useful for finding a missing probability or the chance of something not happening.
Build strong foundations in Probability of Single Events
A free trial class with Teacher Rig helps your Year 8 child master Probability of Single Events now — so IGCSE Maths feels familiar, not frightening, later.
Heading toward IGCSE? See how Probability of Single Events develops in IGCSE Probability (Cambridge 0580) →