The probability scale
Objective. Place likelihoods from impossible to certain on a 0–1 scale.
- 1. Learn
- 2. Worked example
- 3. Practice
- 4. Feedback
- 5. Continue
Learn: the key idea
A probability scale runs from 0 to 1, where 0 means an event is impossible and 1 means an event is certain, with 0.5 representing an event that is equally likely to happen or not happen. Probabilities can be written as fractions, decimals or percentages, all representing the same position on the scale. For an event with equally likely outcomes, probability is calculated as the number of favourable outcomes divided by the total number of possible outcomes. For example, the probability of rolling a 4 on a fair six-sided dice is 1/6, since there is one favourable outcome out of six possible outcomes.
Worked example
A bag contains 5 red counters, 3 blue counters and 2 green counters. Using the probability scale, find the probability of picking a blue counter.
- 1Find the total number of counters: 5+3+2=10.
- 2Identify the number of favourable outcomes: 3 blue counters.
- 3Write the probability as a fraction: 3/10.
- 4Check 3/10 lies between 0 and 1 on the probability scale.
Answer: 3/10
Concept mastery
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Practice
Practice happens on its own screen, one question at a time. Answers stay hidden until you submit yours, and you can stop and pick up at the same question later.
Start practisingCommon mistake. Pupils sometimes give a probability greater than 1 or less than 0, misunderstanding the scale's limits. Always check a calculated probability lies between 0 and 1 inclusive.
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