A price index measures how prices for a defined item or basket change relative to a reference period; learn the formula, weighting methods, and limitations.
A price index is a statistical measure that shows how the prices of a defined item or basket change relative to a reference period. An index converts many prices into one comparable number, often setting the reference period equal to 100. It does not report the basket’s dollar cost or, by itself, explain why prices changed.
An index publisher must make several choices:
These choices determine what the index measures. Two reputable indexes can move differently because they answer different questions.
For a fixed basket using reference-period quantities, a simplified index is:
where (p_{i,t}) is item (i)’s current price, (p_{i,0}) is its reference-period price, and (q_{i,0}) is its reference-period quantity. This is a teaching version of a Laspeyres-type calculation. Official indexes often use sampling, geometric means, quality adjustment, imputation, chained formulas, and updated expenditure weights.
Assume a simple reference basket contains 10 units of food at $2 each and two service visits at $50 each.
| Component | Reference cost | Current cost for the same quantity |
|---|---|---|
| Food | $20 | $24 |
| Services | $100 | $106 |
| Total basket | $120 | $130 |
The basket costs about 8.33% more than in the reference period. Services contribute more to the result because they have the larger basket weight, even though food has the larger percentage increase.
If (I_t) is the current index and (I_{t-k}) is the index (k) periods earlier, the measured price change is:
Suppose an index is 120.0 one year and 123.6 the next. The 12-month inflation rate is 3.0%. The index being above 100 describes cumulative change since the reference period; the 3.0% rate describes change during the latest year.
| Approach | Weight concept | Main strength | Main limitation |
|---|---|---|---|
| Laspeyres-type | Earlier-period quantities or expenditures | Holds the earlier basket broadly constant | Can be slow to reflect substitution toward relatively cheaper items |
| Paasche-type | Current-period quantities or expenditures | Reflects the current spending pattern | Requires current quantity or expenditure data and can understate the cost of maintaining an earlier basket |
| Fisher | Geometric average of Laspeyres and Paasche changes | Uses information from both adjacent periods | More data-intensive and less intuitive |
| Chain-type | Links short-period index changes over time | Updates weights more frequently | Component contributions and long-run additivity can be harder to interpret |
The BEA NIPA methodology explains the Fisher chain-type approach used in U.S. national accounts. The BLS CPI calculation guide documents the more detailed two-stage construction of the U.S. CPI.
| Question | More relevant starting point |
|---|---|
| Prices paid by a defined consumer population | Consumer Price Index |
| U.S. consumption prices within national accounts | PCE Price Index |
| Selling prices received by domestic producers | Producer Price Index |
| Prices of traded raw materials | Commodity Price Index |
| Prices of all domestically produced final output | GDP Deflator |
For a contract, use the exact index named in the agreement. A broad national index may be a poor escalator for a specific material, service, region, or customer group.
This page provides general economic education, not a contractual interpretation, inflation forecast, or personalized financial advice. Consult the current publisher methodology before using an index in a legal or financial calculation.