The consumer price index (CPI) is a statistical index that tracks how the cost of a representative basket of consumer goods and services changes over time. CPI is one of the most common measures of inflation and is often used to adjust wages, pensions, and contracts.
Core Mechanics: A Basket And Weights
A CPI compares the cost of buying the same basket in two periods. A simple (Laspeyres-type) price index uses base-period quantities as weights:
[ CPI_t = 100 \times \frac{\sum_i p_{i,t} q_{i,0}}{\sum_i p_{i,0} q_{i,0}} ]
where (p_{i,t}) is the price of item (i) at time (t), and (q_{i,0}) is the base-period quantity weight.
The weights matter: a housing-heavy basket will respond more to rent changes than a basket with less housing weight.
From CPI To An Inflation Rate
Inflation based on CPI is the percent change in the index:
[ \pi_t = \frac{CPI_t - CPI_{t-1}}{CPI_{t-1}} ]
(Annual inflation is typically the 12-month change for monthly CPI series.)
Headline CPI vs Core Measures
- Headline CPI includes all items in the basket.
- Core inflation uses an index that excludes or down-weights volatile components (often food and energy) to better reflect persistent inflation trends.
Core measures are not “true” inflation and headline “noise”; they are different summaries meant for different questions.
Interpretation And Common Biases
CPI is useful, but it is not a perfect cost-of-living measure.
Common measurement challenges include:
- Substitution bias: when relative prices change, consumers substitute toward cheaper goods; a fixed-weight index can overstate the rise in the cost of achieving the same utility.
- Quality change and new goods: products change over time (better phones, different services), making like-for-like comparisons hard.
- Outlet substitution: consumers shift where they shop (discount vs premium stores), which can change measured prices.
Some statistical agencies use updated weights or chained methods to reduce these issues.
CPI vs Other Price Indices
CPI is often compared with broader measures:
- The GDP deflator covers prices of domestically produced final goods and services, not a consumer basket.
- Producer-side indices focus on prices received by producers rather than prices paid by consumers.