4.2 Different ways of expressing change in index values: one-month change

Percentage changes in prices, costs and quantities over time can be expressed in many different ways; there is no single right way of doing this. Every quantity that measures change has its own specific use. The key thing is to know how to interpret the figures.

To measure one-month change, the index value for a particular month is compared with the corresponding value for the previous month.

For example, the monthly change of the consumer price index from February to March 2003 is calculated as follows:

The consumer price index has gone up from February to March by 0.3 per cent.

Do not calculate the change by deducting the index value for the point of measurement from the value of the previous month, i.e. 105.57 - 105.28 = 0.29. If the figures are close to one hundred the result will be close to the truth, but the procedure is mathematically invalid.

In the consumer price index, changes are always calculated from figures truncated to two decimal places (truncated figures are simply cut off after two decimals rather than being rounded).

For example, 104.568321 = 104.56.