Spots and Measles

Age 11 to 16
Challenge Level

This resource is part of the collection Probability and Evidence.

99% of people who have measles have spots. If Ben has red spots, how likely do you think it is it that he has measles?

Here are three pieces of information that might help you to work out this probability:
  • There are about 12 million children in the UK.
  • About 1,200 of these get measles every year.
  • Roughly 10% of the 12 million children suffer from spots.

How might you be able to use this information to be able to calculate the probability that Ben has measles?
Are there any tables or diagrams that might help you represent this information?

Click below to see a possible method for organising the information:

You could draw a two-way (or contingency) table like this.

  Measles Not Measles Total
No Spots      
Total     12 000 000

Can you use the information to complete the table?

Can you now work out the probability that Ben has measles?

Are there any other factors that might affect the probability that Ben has measles?
How could you factor these into the calculations?