Index numbers

Economists frequently use index numbers when making comparisons over time. An index starts in a given year, the base year, at an index number of 100. In subsequent years, percentage increases push the index number above 100, and percentage decreases push the figure below 100. An index number of 102 means a 2% rise from the base year, and an index number of 98 means a 2% fall.

Using an index makes quick comparisons easy. For example, when comparing house prices from the base year of 2012, an index number of 110 in 2013 indicates an increase in house prices of 10% in 2013.

House price index

YearAverage house price (£000)% changeIndex number
2012200Base year100
201322010110
201423015115
20152105105
2016195-2.4397.57

An index number of 97.57 in 2016 means that during 2016 house prices fell by 2.43% from the base year of 2012.