| The value of a flow of goods or services can change from one period to another because the prices of the goods and services involved change, and/or because the volume of goods and services changes. The change in volume is only made apparent by calculating the flows of goods and services at so-called constant prices. The constant price figures express the value of the goods and services at the price that applied in a previous period. The change in volume is specified by a volume index. The volume index expresses the percentage change in flows of goods and services expressed at constant prices from one period to another. Volume in this context means in principle both the volume or number of units of goods and services involved and also their quality. |