called the base value, which is usually set at 100. It's calculated as 100 times the ratio of the current value to the base--for example, if a commodity's price doubles from 1960 to 1970, its index number would be 200 with 1960 as the base. Index numbers help simplify complex data about business activity, cost of living, or employment into numbers that are easy to understand and compare over time.

Some index numbers, called superlative index numbers, are designed to closely approximate an