Bureau of Labor Statistics
Mid-Atlantic Information Office
Suite 610 EastThe Curtis Center
170 South Independence Mall West
Philadelphia, PA 19106-3305
Consumer Price Index (CPI)
Explanatory Note (PDF)
Consumer Price Index
The Consumer Price Index (CPI) is a measure of the average change in prices over time of goods and services purchased by households. The Bureau of Labor Statistics publishes CPIs for two population groups: (1) the CPI for Urban Wage Earners and Clerical Workers (CPI-W), which covers households of wage earners and clerical workers that comprise approximately 32 percent of the total population and (2) the CPI for All Urban Consumers (CPI-U) and the Chained CPI for All Urban Consumers (C-CPI-U), which cover approximately 87 percent of the total population and include, in addition to wage earners and clerical worker households, groups such as professional, managerial, and technical workers, the self-employed, short-term workers, the unemployed, and retirees and others not in the labor force.
The CPIs are based on prices of food, clothing, shelter, and fuels, transportation fares, charges for doctors' and dentists' services, drugs, and other goods and services that people buy for day-to-day living. Prices are collected in 87 urban areas across the country from about 50,000 housing units and approximately 23,000 retail establishments--department stores, supermarkets, hospitals, filling stations, and other types of stores and service establishments. All taxes directly associated with the purchase and use of items are included in the index. Prices of fuels and a few other items are obtained every month in all 87 locations. Prices of most other commodities and services are collected every month in the three largest geographic areas and every other month in other areas. Prices of most goods and services are obtained by personal visits or telephone calls of the Bureau's trained representatives.
In calculating the index, price changes for the various items in each
location are averaged together with weights, which represent their
importance in the spending of the appropriate population group. Local
data are then combined to obtain a U.S. city average. For the CPI-U and
CPI-W separate indexes are also published by size of city, by region of
the country, for cross-classifications of regions and population-size
classes, and for 26 local areas. Area indexes do not measure differences
in the level of prices among cities, they only measure the average change
in prices for each area since the base period. For the C-CPI-U data are
issued only at the national level. It is important to note that the CPI-U
and CPI-W are considered final when released, but the C-CPI-U is issued in
preliminary form and subject to two annual revisions.
The index measures price change from a designed reference date. For
the CPI-U and the CPI-W the reference base is 1982-84 equals 100.0. The
reference base for the C-CPI-U is December 1999 equals 100.
An increase of 16.5 percent from the reference base, for example, is shown
as 116.5. This change can also be expressed in dollars as follows: the
price of a base period market basket of goods and services in the CPI has
risen from $10 in 1982-84 to $11.65.
Calculating Index Changes
Movements of the indexes from one month to another are
usually expressed as percent changes rather than changes in index
points, because index point changes are affected by the level of
the index in relation to its base period while percent changes
are not. The example in the accompanying box illustrates the
computation of index point and percent changes.
Percent changes for 3-month and 6-month periods are
expressed as annual rates and are computed according to the
standard formula for compound growth rates. These data indicate
what the percent change would be if the current rate were
maintained for a 12-month period.
|Index Point Change
|Less previous index
|Equals index point change
|Index point difference
|Divided by the previous index
4.116 / 198.3
|Results multiplied by one hundred
0.021 x 100
|Equals percent change
Last Modified Date: May 16, 2008