9.1 Probability scales

In real life, things usually fall somewhere in between the two extremes of ‘will never happen’ and ‘will definitely happen’.

We can use a probability scale to measure how likely events are to occur:

Figure 13 A probability scale
  • The probability of an impossible event (‘will never happen’) is 0.
  • The probability of a certain event (and ‘will definitely happen’) is 1.
  • All other events come between 0 and 1.
  • Events with an even chance have a probability of one divided by two.

Now try the following activity, where you’ll need a ruler and a pencil. Remember to check your answers once you have completed the questions.

Activity 15: Looking at probability

Use a ruler to draw your own probability scale. Mark on it 0, 1/2 and 1. Label 0 as ‘impossible’ and 1 as ‘certain’.

Then mark these statements on the probability scale with crosses and label them with their question letter:

  1. The probability that the sun will rise tomorrow.
  2. The probability that you will run the London Marathon next year.
  3. The probability of dying one day.

Answer

Here are the answers:

Figure 14 A probability scale (answer)

Of course, if you are a long-distance runner or plan to be one, your location for (b) might be closer to 1!

Summary

In this section you have:

  • learned about the possibility of different events happening
  • shown that some events are more likely to occur than others.

OpenLearn - Everyday maths 1
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