index number

A statistical device used mainly in economic and financial practice for tracking changes in some activity over time. Most start from a base of 100.

Index number, in econometrics, is a figure reflecting a change in value or quantity as compared with a standard or base. The base usually equals 100 and the index number is usually expressed as a percentage. For example, if a commodity cost twice as much in 1970 as it did in 1960, its index number would be 200 relative to 1960. Index numbers are used especially to compare business activity, the cost of living, and employment.