In economics, way of showing changes in a series of variables by picking one value as the base value, calling it 100, and then adjusting the other values in proportion to the base value. For example, the income of three individuals may be Ł5,000, Ł10,000, and Ł15,000. Taking Ł10,000 as the base value, the three incomes expressed in index number form would be 50, 100, and 150. If Ł5,000 were taken as the base value, the three index numbers would be 100, 200, and 300. Index numbers are particularly useful for showing changes over time.