The Probability Scale

In a Nutshell

Probability measures how likely something is to happen, on a scale from 0 (impossible) to 1 (certain).

Every event has a probability — a number that tells us how likely it is. We place events on a probability scale that runs from 00 to 11.

An event with probability 00 is impossible — it can never happen. An event with probability 11 is certain — it must happen. Everything else sits somewhere between.

We also use special words: unlikely (close to 00), even chance (exactly 0.50.5), and likely (close to 11).

Probabilities can be written as fractions, decimals or percentages. For example, an even chance is 12\tfrac{1}{2}, 0.50.5, or 50%50\%.

Probability scale from 0 to 1 A horizontal number line from 0 to 1, divided into regions labelled Impossible, Unlikely, Even chance, Likely and Certain. A draggable marker shows where an event sits on the scale. 0 Impossible 0.25 Unlikely 0.5 Even chance 0.75 Likely 1 Certain 0.5

Watch it work

Question: Place these events on the probability scale: (a) rolling a 7 on a normal die, (b) it rains some time this year in London, (c) flipping heads on a fair coin.

Have a go

Q1. Where on the probability scale would you place "picking a red ball from a bag that contains only blue balls"?

Q2. A bag contains 3 red sweets and 3 green sweets. What word describes the probability of picking a red sweet?

Q3. A weather forecast says the chance of rain tomorrow is 0.90.9. Is rain likely or unlikely?

Q4. Put these probabilities in order from least likely to most likely: 0.750.75, 0.10.1, 0.50.5, 11.