1. What is the CPI?
The Consumer Price Index (CPI) is a measure of the average change overtime in the prices paid by urban consumers for a market basket of consumer goods and services.
2. How is the CPI market basket determined?
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 24,000consumers 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 12,000consumers 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 approximately24,000 weekly diaries and 48,000 quarterly interviews used to determine the importance, or weight, of the item categories in the CPI index structure.
3. How is the CPI sample created?
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.
4. How is the CPI calculated?
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.
Additional information on how the CPI is calculated is available in the CPI section of the BLS Handbook of Methods.
5. How is the CPI used?
The CPI affects nearly all Americans because of the many ways it is used. Some examples of how it is used follow:
As an economic indicator. The CPI is the most widely used measure of inflation and is sometimes viewed as an indicator of the effectiveness of government economic policy. It provides information about price changes in the Nation's economy to government, business, labor, and private citizens and is used by them as a guide to making economic decisions. In addition, the President, Congress, and the Federal Reserve Board use trends in the CPI to aid in formulating fiscal and monetary policies.
As a deflator of other economic series. The CPI and its components are used to adjust other economic series for price changes and to translate these series into inflation-free dollars. Examples of series adjusted by the CPI include retail sales, hourly and weekly earnings, and components of the National Income and Product Accounts.
The CPI is also used as a deflator of the value of the consumer's dollar to find its purchasing power. The purchasing power of the consumer's dollar measures the change in the value to the consumer of goods and services that a dollar will buy at different dates. In other words, as prices increase, the purchasing power of the consumer's dollar declines.
As a means of adjusting dollar values. The CPI is often used to adjust consumers' income payments (for example, Social Security), to adjust income eligibility levels for government assistance, and to automatically provide cost-of-living wage adjustments to millions of American workers. As a result of statutory action, the CPI affects the income of millions of Americans. Over 50 million Social Security beneficiaries, and military and Federal Civil Service retirees, have cost-of-living adjustments tied to the CPI.
Another example of how dollar values may be adjusted is the use of the CPI to adjust the Federal income tax structure. These adjustments prevent inflation-induced increases in tax rates. In addition, eligibility criteria for millions of food stamp recipients, and children who eat lunch at school, are affected by changes in the CPI. Many collective bargaining agreements also tie wage increases to the CPI.
6. Whose buying habits does the CPI reflect?
The CPI reflects spending patterns for each of two population groups: all urban consumers and urban wage earners and clerical workers. The all urban consumer group represents about 93 percent of the total U.S. population. It is based on the expenditures of almost all residents of urban or metropolitan areas, including professionals, the self-employed, the unemployed, and retired people, as well as urban wage earners and clerical workers. Not included in the CPI are the spending patterns of people living in rural nonmetropolitan areas, those in farm households, people in the Armed Forces, and those in institutions, such as prisons and mental hospitals. Consumer inflation for all urban consumers is measured by two indexes, namely, the Consumer Price Index for All Urban Consumers (CPI-U) and the Chained Consumer Price Index for All Urban Consumers (C-CPI-U).
The Consumer Price Index for Urban Wage Earners and Clerical Workers (CPI-W) is based on the expenditures of households included in the CPI-U definition that also meet two additional requirements: more than one-half of the household's income must come from clerical or wage occupations, and at least one of the household's earners must have been employed for at least 37 weeks during the previous 12 months. The CPI-W population represents about 29 percent of the total U.S. population and is a subset of the CPI-U population.
The CPI does not necessarily measure your own experience with price change. It is important to understand that BLS bases the market baskets and pricing procedures for the CPI-U and CPI-W populations on the experience of the relevant average household, not of any specific family or individual. For example, if you spend a larger-than-average share of your budget on medical expenses, and medical care costs are increasing more rapidly than the cost of other items in the CPI market basket, your personal rate of inflation may exceed the increase in the CPI. Conversely, if you heat your home with solar energy, and fuel prices are rising more rapidly than other items, you may experience less inflation than the general population does. A national average reflects millions of individual price experiences; it seldom mirrors a particular consumer's experience.
Additional information on this topic is available in the Why the Published Averages Don’t Always Match an Individual’s Inflation Experience factsheet.