price index, measure of relative price changes, consisting of a series of numbers arranged so that a comparison between the values for any two periods or places will show the average change in prices between periods or the average difference in prices between places. Price indexes were first developed to measure changes in the cost of living in order to determine the wage increases necessary to maintain a constant standard of living. They continue to be used extensively to estimate changes in prices over time and are also used to measure differences in costs among different areas or countries. See also consumer price index; wholesale price index.
The central problem of price-data collection is to gather a sample of prices representative of the various price quotations for each of the commodities under study. Sampling is almost always necessary. The larger and the more complex the universe of prices to be covered by the index, the more complex the sampling pattern will have to be. An index of prices paid by consumers in a large and geographically varied country, for example, ideally should be based on a sample representative of price changes in different cities and localities, in different types of outlets (supermarkets, department stores, neighbourhood shops, etc.), and for different commodities. The number of prices chosen to represent each type of city (or metropolitan area), type of outlet, and category of commodity would ideally be proportionate to its relative importance in the expenditures of the nation. Most price indexes are based on some approximation to such a sampling design.
Once the commodity sample has been chosen, the collection of prices must be planned so that differences between the prices of any two dates will reflect changes in price and price alone. Ideally one would collect the prices of exactly the same items at each date. To this end, commodity prices are sometimes collected in accordance with detailed specifications such as “wheat, no. 2 red winter, bulk, carlots, f.o.b. Chicago, spot market price, average of high and low, per bushel.” If all commodities were as standardized as wheat, the making of price indexes would be much simpler than it is. In fact, except for a limited range of goods consisting mainly of primary products, it is very difficult to describe a product completely enough so that different pricing agents can go into stores and price an identical item on the basis of description alone. In view of this difficulty, price-collection agencies sometimes rely upon each respondent, usually a business firm, to report prices in successive periods for the same variant of a product (say, men’s shoes); the variant chosen by each respondent may be different, but valid data will be obtained as long as each provides prices for the same variant he originally chose. Because a product may vary in quality from one observation to another, even though it retains the same general specification, the usual procedure is to avoid the computation of average observed prices for each commodity for each date. Instead, each price received from each source is converted to a percentage of the corresponding price reported for the previous period from the same source. These percentages are called “price relatives.”
The next step is to combine the price relatives in such a way that the movement of the whole group of prices from one period to another is accurately described. Usually, one begins by averaging the price relatives for the same specification (e.g., men’s high work shoes, elk upper, Goodyear welt, size range 6 to 11) from different reporters. Sometimes separate averages for each commodity are calculated for each city, and the city averages are combined.
A more difficult problem arises in combining the price relatives for different commodities. They must be given different weights, of course, because not all the commodities for which the prices or price relatives have been obtained are of equal importance. The price of wheat, for example, should be given more weight in an index of wholesale prices than the price of pepper. The difficulty is that the relative importance of commodities changes over time. Some commodities even drop out of use, while new ones appear, and often an item changes so much in composition and design that it is doubtful whether it can properly be considered the same commodity. Under these conditions, the pattern of weights selected can be accurate in only one of the periods for which the index numbers have been calculated. The greater the lapse of time between that period and other periods in the index, the less meaningful the price comparisons become. Price indexes thus can give relatively accurate measures of price change only for periods close together in time.