A consumer price index (CPI) is a statistical measure of changes over time in the prices paid by households for a representative basket of goods and services. Published by national statistical agencies, it is a principal indicator of inflation experienced through consumer expenditure. A CPI measures price movement rather than the monetary cost of the basket itself: its level is normally expressed relative to an index reference period assigned a value of 100. Its population coverage, expenditure boundaries, and calculation methods depend on the particular index. (bls.gov)
Coverage and expenditure weights
A CPI generally covers consumption categories such as food, housing services, clothing, transport, recreation, and communications. Its scope distinguishes consumption from investment: purchases of financial assets are not consumer expenditure. In the United States, prices include sales and excise taxes directly associated with purchases, but income taxes are excluded. Treatment of insurance, subsidized services, and owner-occupied housing requires specific methodological decisions. (bls.gov)
The representative consumer basket is not a literal shopping list used by every household. It is a statistical structure containing product categories, sampled items, and expenditure weights. Weights reflect the relative importance of categories in aggregate consumer spending, commonly estimated from household expenditure surveys and other sources. Consequently, a price increase in a heavily weighted category affects the overall index more than an equal increase in a lightly weighted category. Baskets and weights are periodically updated to reflect changing expenditure patterns. (imf.org)
Construction and index formulas
Compilers select representative locations, outlets, and products, then collect comparable prices over successive periods. Complete observation of every consumer transaction is generally impractical, making sampling a central part of CPI statistics. Prices for narrowly defined groups are first combined into elementary indexes; these are then aggregated using expenditure weights. (elibrary.imf.org)
A simple fixed-basket model is the Laspeyres price index:
[ I_t=100\frac{\sum_i p_{it}q_{i0}}{\sum_i p_{i0}q_{i0}}, ]
where (p_{it}) is the price of item (i) in period (t), and (q_{i0}) is its quantity in the base period. It compares the cost of unchanged base-period quantities at current and base-period prices. Actual CPIs often use more elaborate arrangements: the price-reference, weight-reference, and index-reference periods need not coincide. (imf.org)
No single formula describes every national CPI. The United States uses geometric means for most elementary indexes and modified Laspeyres aggregation for its principal CPI measures. Its chained CPI incorporates expenditure information from successive periods to account more fully for substitution across categories, with preliminary values revised as expenditure data become available. (bls.gov)
Reading inflation rates
For index values (I_t) and (I_{t-k}), the percentage price change is
[ 100\left(\frac{I_t}{I_{t-k}}-1\right). ]
For a monthly series, (k=1) gives the month-to-month change and (k=12) the year-over-year change. A hypothetical increase from 120 to 126 is five percent, not six percent: index-point changes and percentage changes are different quantities. Rebasing changes the index’s displayed level but not its percentage movements. (bls.gov)
Seasonal adjustment removes estimated recurring seasonal movements from a time series, helping distinguish short-term developments from regular patterns. Seasonally adjusted CPI values can be revised when seasonal factors are re-estimated. Core inflation measures exclude selected components—commonly food and energy—to provide another view of price developments; those components remain in the all-items CPI. (bls.gov)
A declining positive inflation rate represents disinflation, not necessarily falling prices. Deflation means a decline in the general price level. Thus, slower CPI growth can coexist with prices remaining substantially above earlier levels. These distinctions follow from separating changes in an index’s level from changes in its growth rate. (bls.gov)
Quality change and cost of living
Products change, disappear, and acquire new features. Quality adjustment attempts to separate price changes from changes in what consumers receive. One technique, hedonic regression, estimates the contribution of measurable product characteristics to price. Such adjustments aim to compare equivalent quality rather than treating every difference between replacement products as inflation. (bls.gov)
A CPI is related to, but not identical with, a cost-of-living index. The latter asks how much expenditure is needed to maintain a specified level of utility or living standard. Consumers may respond to relative price changes by switching products; a rigid basket cannot fully capture that substitution. Moreover, consumer well-being depends on environmental conditions and public goods that conventional CPIs do not comprehensively measure. (bls.gov)
Uses and comparisons
CPIs inform monetary policy and economic analysis. They also support indexation of wages, pensions, benefits, rents, and tax thresholds. Dividing a nominal monetary series by an appropriately scaled CPI expresses it in constant purchasing-power terms, although the suitability of CPI depends on the expenditure being analyzed. (imf.org)
Different price indexes answer different questions. The GDP deflator concerns domestic production rather than household consumption; unlike CPI, it encompasses investment goods and exports while excluding imports. In the European Union, the Harmonised Index of Consumer Prices provides comparable consumer inflation measures under a common framework. Housing treatment remains an important distinction: the US CPI estimates owner-occupied shelter through owners’ equivalent rent, while the HICP excludes owner-occupied housing costs from its coverage. (bea.gov)
Finally, aggregate CPI growth need not match an individual household’s experience. Spending patterns, location, housing tenure, and prices actually paid vary. CPI levels also cannot establish which country is more expensive, because national index reference levels are conventional rather than comparable measures of absolute prices. (bls.gov)