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. Percent Change
To calculate percent change you would use the following formula:
Change = New  Original
Page last updated on 20/10/13
