Gasoline, unleaded regular in Mountain, all urban consumers, not seasonally adjusted

The Consumer Price Index (CPI) is a statistical measure of change, over time, of the prices of goods and services in major expenditure groups — such as food, housing, apparel, transportation, and medical care — typically purchased by urban consumers. Essentially, it compares the cost of a sample "market basket" of goods and services in a specific month relative to the cost of the same "market basket" in an earlier reference period, known as the base period.

The revised CPI introduced by the BLS in 1998 includes indexes for two populations: urban wage earners and clerical workers (CW), and all urban consumers (CU). Both the CW and the CU utilize updated expenditure weights based upon data tabulated from three years (1982, 1983, and 1984) of the Consumer Expenditure Survey and incorporate a number of technical improvements, including an updated and revised item structure. To construct the two indexes, prices for about 100,000 items and data on about 8,300 housing units are collected in a sample of 91 urban places.

Comparison of indexes for individual CMSAs or cities shows only the relative change over time in prices between locations. These indexes cannot be used to measure interarea differences in price levels or living costs. U.S. average indexes for both populations are available for about 305 consumer items and groups of items. In addition, over 100 of the indexes have been adjusted for seasonality. The indexes are monthly with some beginning in 1913. Semi-annual indexes have been calculated for about 100 items for comparison with semi-annual areas. Semi-annual indexes are available from 1984 forward.

Area indexes for both populations are available for 26 urban places, with indexes published for about 42 items and groups per area. The indexes are published monthly for three areas, bimonthly for eleven areas, and semi-annually for 12 urban areas. Regional indexes for both populations are available for four regions with about 55 items and groups per region; these are monthly from January 1987 forward, with some beginning as early as 1966. City-size indexes for both populations are available for three size classes with about 55 items and groups per class, and are monthly from January 1987 with most beginning in 1977. Region/city-size indexes cross-classify by region and city-size class, providing about 42 items and groups for each of 13 cross calculations, monthly from January 1987.

Annual averages are available for all unadjusted series in the CW and CU. Most indexes have a base period of 1982-1984 = 100; other indexes added with the 1998 revision are based more recently. The base period value is 100.0, except for the "Purchasing Power" values (AAOR and SAOR) where the base period value is 1.000. Indexes are stored to one decimal place, except for the "Purchasing Power" values which are stored to three decimal places. Updates become available near the 15th of the month following the reference month.

References: BLS Handbook of Methods, Chapter 17, "Consumer Price Index", BLS Bulletin 2285, April 1988.