Index numbers, especially in economics, are used to help make comparisons between a series of very large numbers. A selected point is given the value of 100 and others are compared to it. The point which is given the value of 100 is referred to as the base number.

As an example, assume the GDP of a country in year 1 was £400 million, and in year 10 it was £600 million. If year 1 is made the base year, then the value of the index in year 10 would be 150. This is because £600 million is 150% of £400 million.