An index is a statistical measure of variation in a representative group of individual data points. An index number is an economic data figure reflecting price or quantity compared to a standard or base value. Derived from a number of sources, company performance, prices, productivity, and employment, economic indices track health from different perspectives. The best-known index number is the consumer price index that measures changes in retail prices paid by consumers. Index numbers are a useful way of expressing economic data time series and comparing or contrasting information.