Maths · Statistics & Probability
Probability scale
Place events on a 0-1 probability scale.
What is a probability scale?
A probability scale runs from 0 to 1. Zero means impossible; one means certain. Everything in between shows how likely an event is. The closer to 1, the more likely it happens.
Key points on the scale
Impossible events have probability 0. Certain events have probability 1. Equally likely events (like heads or tails) sit at 0.5, exactly halfway. Very unlikely events sit close to 0.
Writing probabilities
Probabilities can be fractions, decimals or percentages. For example, 1/2 equals 0.5 equals 50%. All three forms describe the same chance. On a 0–1 scale we usually use decimals.
Comparing events
If event A has probability 0.2 and event B has probability 0.7, then B is more likely. The bigger the number, the more likely the event. Use the scale to rank events.
Worked examples
1. A fair coin is flipped. Where does 'getting heads' sit on a 0–1 probability scale?
- A fair coin has two equally likely outcomes: heads or tails.
- So the probability of heads is 1 out of 2, which is 1/2.
- Convert 1/2 to a decimal: 1 ÷ 2 = 0.5.
- The event sits at 0.5 on the scale, exactly halfway between impossible and certain.
2. Rolling a standard dice and getting a 7. Where does this sit on the scale?
- A standard dice has faces numbered 1 to 6 only.
- It is impossible to roll a 7.
- Impossible events have probability 0.
- This event sits at 0 on the probability scale.
3. The sun will rise tomorrow. Where does this event sit on the scale?
- The sun rising tomorrow is practically certain based on all evidence.
- Certain events have probability 1.
- This event sits at 1 on the probability scale.
Try it yourself
A bag holds 3 red marbles and 1 blue marble. You pick one at random. Where does 'picking red' sit on the 0–1 scale?
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