Indices - indices are used to make comparisons over time or geographical space. For example, price indices measure developments in the general price level over time. In order to reveal general trends, the indices are calculated as a weighted average of the price changes for different goods and services. Purchasing power indices (such as the PPP) measure differences in different countries' price levels at a given time.

Indices are often given a value of 100 to provide a basis for comparison, the so-called base of the index. For example, price indices that measure changes over time will have a value of100 in a specific year, so that the value of the index in any period expresses the change in prices relative to that year. The price change can also be calculated between any two periods by comparing the values of the index in these two periods. Purchasing power indices, which measure geographical price differences, will have one country or geographical region with a base index of 100. Another country's index value will then express that country's price level relative to the base country.