Cyclopedia
Consumer Price Index (CPI)
Summary
The Consumer Price Index (CPI) is a measure of the average change over time in the prices paid by urban consumers for a market basket of consumer goods and services.
General Information on the Consumer Price Index
[Derived from the Information from the Bureau of Labor Statistics]
The CPI market basket is developed from detailed expenditure information provided by families and individuals on what they actually bought. There is a time lag between the expenditure survey and its use in the CPI. For example, CPI data in 2016 and 2017 was based on data collected from the Consumer Expenditure Surveys for 2013 and 2014. In each of those years, about 24,000 consumers from around the country provided information each quarter on their spending habits in the interview survey. To collect information on frequently purchased items, such as food and personal care products, another 12,000 consumers in each of these years kept diaries listing everything they bought during a 2-week period.
Over the 2 year period, then, expenditure information came from approximately 24,000 weekly diaries and 48,000 quarterly interviews used to determine the importance, or weight, of the item categories in the CPI index structure.
A particular item enters the CPI sample through a process called initiation. This initiation process, typically carried out in person by a CPI data collector, involves selecting a specific item to be priced from the category that has been designated to be priced at that store. For example, suppose a particular grocery store has an outlet where cheese will be priced. A particular type of cheese item will be chosen, with its likelihood of being selected roughly proportional to its popularity. If, for example, cheddar cheese in 8 oz. packages makes up 70 percent of the sales of cheese, and the same cheese in 6 oz. packages accounts for 10 percent of all cheese sales, and the same cheese in 12 oz. packages accounts for 20 percent of all cheese sales, then the 8 oz. package will be 7 times as likely to be chosen as the 6 oz. package. After probabilities are assigned, one type, brand, and container size of cheese is chosen by an objective selection process based on the theory of random sampling. The particular kind of cheese that is selected will continue to be priced each month in the same outlet.
This item will be repriced, monthly or bimonthly, until it is replaced after four years through sample rotation. Repricing is usually done in person, but may be done via telephone or the internet. The process of selecting individual quotes results in the sample as a whole containing a wide variety of specific items of a category roughly corresponding to consumer purchases. So the cheese sample (or the new vehicle sample, the television sample, etc.) contains a wide variety of styles and brands of cheese, vehicles, televisions, etc.
The CPI is a product of a series of interrelated samples. First, using data from the U.S. Census we select the urban areas from which data on prices are collected. Next, another sample (of about 14,500 families each year) serves as the basis for a Telephone Point-of-Purchase Survey (TPOPS) that identifies the places where households purchase various types of goods and services, forming the basis for the CPI outlet sample. Using data from the Consumer Expenditure Survey, BLS statisticians assign quotes in the CPI item categories to specific outlets. A specific item is then chosen for selection using a process which bases the probability of selection for an item on the share the item composes within the outlet’s revenue in that item category.
Recorded price changes are weighted by the importance of the item in the spending patterns of the appropriate population group. The combination of carefully selected geographic areas, retail establishments, commodities and services, and associated weight, gives a weighted measurement of price change for all items in all outlets, in all areas priced for the CPI.
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GAO Glossary of Terms and Definition (September 2005)
Consumer Price Index (CPI) (Economics Term)
A measure of the average change over time in the prices paid by urban consumers for a market basket of consumer goods and services commonly referred to as “inflation.” Measures for two population groups are currently published, CPI-U and CPI-W. CPI- U is based on a market basket determined by expenditure patterns of all urban households, while the market basket for CPI-W is determined by expenditure patterns of only urban wage-earner and clerical-worker families. The urban wage- earner and clerical-worker population consists of clerical workers, sales workers, craft workers, operatives, service workers, and laborers. Both indexes are published monthly by the Bureau of Labor Statistics. The CPI is used to adjust for inflation, the income payments of Social Security beneficiaries, and payments made by other programs. In addition, the CPI is used to adjust certain amounts defined by the tax code, such as personal exemptions and the tax brackets.
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