A statistic which assigns a single number to several individual statistics in order to quantify trends. The best-known index in the United
States is the consumer price index, which gives a sort of "average" value
for inflation based on price changes for a group of selected products. The Dow Jones
and NASDAQ indexes for the New York and American Stock Exchanges, respectively, are
also index numbers.

Let be the price per unit in period , be the quantity produced in period , and be the value of the units. Let be the estimated relative importance of a product. There
are several types of indices defined, among them those listed in the following table.