Index Calculations

Index numbers are used to show relative values – a change in a variable relative to a base point in time. Index data uses a base (usually 100). Index numbers have no units.

If you wanted to calculate the percentage change from the base you would just subtract 100, e.g. Year 1 = 100, Year 3 = 110. The percentage change from year 3 to year 12 is 10%. If you were doing it from a year that doesn’t have an index of 100 then you would have to use the percentage change formula.

To calculate an index number you would use the formula

To calculate the percentage change of index values you would use the formula 

If you have to variables on a graph with the same index year but not necessarily the same index value, then it is important to be careful when making comparisons. Meaningful comparisons can only be made about the percentage change of both; not actual values.