Population coverage

For each published CPI data series, two separate indexes are available: All Urban Consumers (CPI-U) and Urban Wage Earners and Clerical Workers (CPI-W). Both the CPI-U and the CPI-W reflect only the buying habits of urban consumers. The CPI-U is the most comprehensive of the two and represents the expenditures by all urban consumers, about 87 percent of the total U.S. population.

The CPI-W represents a subset of the CPI-U population, that is, the expenditures by urban households that derive more than one-half of their income from clerical or hourly wage occupations. These households comprise about 32 percent of the total U.S. population.

Reference Base

The CPI is a tool that simplifies the measurement of changes in prices over time. By selecting an appropriate reference base and setting the average index level for that time period equal to 100, it is possible to compare this month’s (or last year’s) price index level with the reference base period or to any other time period. The current standard reference base period is 1982-84=100. That is, all price changes are measured from a base (100) that represents the average index level of the 36-month period encompassing 1982, 1983, and 1984.

Prior to the release of the CPI for January 1988, the standard reference base was 1967=100. As a service to our customers with existing escalation provisions, BLS continues to publish CPI all items indexes for the U.S. city average and all local areas using the old base. Note that, although comparisons cannot be made between indexes with different reference bases, the conversion to a new reference base does not affect the measurement of percent changes in a given index series from one time period to another, except for rounding differences.

In addition, BLS publishes several index series with a reference base more recent than January 1982. These indexes either could not be rebased because historical price data were not available for the entire 1982-84 reference base period or they represent items (such as mid-grade gasoline) that only recently were introduced into the CPI.

The CPI market basket

The CPI market basket represents all the consumer goods and services purchased by urban households. Price data are collected for over 180 categories, which BLS has grouped into 8 major groups. These major groups, with examples of categories in each, are as follows:

• Food and beverages (ham, eggs, carbonated drinks, coffee, meals and snacks);

• Housing (rent of primary residence, fuel oil, bedroom furniture);

• Apparel (men’s shirts and sweaters, women’s dresses, jewelry);

• Transportation (new vehicles, gasoline, tires, airline fares);

• Medical care (prescription drugs and medical supplies, physicians’ services, eyeglasses and eye care, hospital services);

• Recreation (television sets, cable TV, pets and pet products, sports equipment, admissions);

• Education and communication (college tuition, postage, telephone services, computer software and accessories);

• Other goods and services (tobacco and smoking products, haircuts and other personal care services, funeral expenses).

Indexes for all of the above categories are published at the U.S. city average level. Due to limitations in sample size, however, many of the smaller expenditure categories are not available at regional and local area levels. Instead, related categories are aggregated and published as part of a more comprehensive category. For example, physicians’ services and eyeglasses and eye care are combined with similar categories and published as professional medical services at the regional level. At the metropolitan area level, professional medical services is, in turn, combined further and published as medical care.

Seasonal adjustment

Many of the goods and services included in the CPI market basket exhibit "seasonal" patterns of price movement. BLS factors out the seasonal trends from the underlying change in prices and publishes the resulting seasonally adjusted price indexes for those goods and services that display consistent seasonal patterns of price change.

BLS publishes seasonally adjusted indexes only at the U.S. city average level (for both the CPI-U and CPI-W). They are not available for regional or local area levels. Seasonally adjusted indexes are not appropriate for use in escalation or cost-of-living adjustments (COLAs) because what consumers actually pay for goods and services is represented by the unadjusted data.

Additional information

Additional information on the CPI can be found on the CPI Internet web site (http://www.bls.gov/cpi/). You may also contact any of the eight BLS regional offices (located in Boston, New York, Philadelphia, Atlanta, Chicago, Dallas, Kansas City, and San Francisco); or call our national information staff at (202) 691-7000.

Information in this report is in the public domain and, with appropriate credit, may be reproduced without permission. This information is available to sensory impaired individuals upon request. Voice phone: (202) 691-5200; Federal Relay Service: 1-800-877-8339.

Last Modified Date: October 16, 2001