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Consumer Price Index
Bureau of Labor Statistics > Consumer Price Index > Methods > Seasonal Adjustment

Intervention Analysis in Seasonal Adjustment

Seasonal adjustment is used to identify and factor out seasonal movements in some Consumer Price Index (CPI) series. The resultant seasonally adjusted data reflect an estimate of non-seasonal price movements.  The CPI program uses the Census Bureau’s X-13ARIMA-SEATS software to calculate factors for seasonal adjustment of both historical and current series. 

Seasonal adjustment for certain CPI series includes a technique called intervention analysis seasonal adjustment (IASA) that is part of the Census X-13 seasonal adjustment program. The seasonal factors produced for CPI series using IASA more accurately represent the underlying seasonal pattern, because it allows economic phenomena that are not seasonal in nature, such as outliers and level shifts, to be factored out of indexes before the calculation of seasonal factors.  An outlier is an extreme value for a particular month.  A level shift is a change or shift in the price level of a CPI series caused by an event, such as a sales tax increase or oil embargo, occurring over one or several months.  The result is an adjustment based on a representation of the series with the seasonal pattern emphasized.  IASA also makes it possible to account for seasonal shifts, resulting in a better seasonal adjustment in the periods before and after the shift occurred. 

X-13ARIMA-SEATS’s built-in regression variables are used for directly estimating the effects of sudden level changes and other disruptions and removing those effects before the calculation of seasonal factors. Seasonal factors are applied to the original unadjusted series without interventions removed. As a result, level shifts and outliers, removed for the calculation of seasonal factors, are present in the seasonally adjusted series.

For a comprehensive discussion of IASA, see "Improvements to CPI Procedures for Intervention Analysis Seasonal Adjustment" in the December 1996 issue of the CPI Detailed Report.

This year, BLS examined CPI series using the 8-year span from January 2015 through December 2022 with some exceptions. Fuel oil and other fuels was examined using a 7-year span from January 2016 through December 2022. The following series were examined using a 5-year span from January 2018 through December 2022: Used cars and trucks, New vehicles, New cars and New trucks.

In April 2022, the Consumer Price Index program replaced the survey data collected by the BLS for the New vehicles index with transaction data from J.D. Power and changed its estimation methodology from a geometric average to an expenditure weighted Tornqvist formula. This index continues to include prices for cars and trucks, but no longer includes motorcycle prices. Two special relative series, New cars and New trucks, are also based on J.D. Power data. The 2023 seasonal adjustment factors for New Vehicles, New cars and New trucks were determined using five years of estimated price change under the new methodology. The data used to determine seasonal factors for these series are available on the CPI seasonal adjustment web page.

In January 2023, BLS adjusted the series listed below using IASA techniques:

  • Airline fares

  • Appliances

  • Beef and veal

  • Canned fruits and vegetables

  • Canned vegetables

  • Car and truck rental

  • Carbonated drinks

  • Cereals and bakery products

  • Cereals and cereal products

  • Crackers, bread and cracker products

  • Durables

  • Flour and prepared flour mixes

  • Fresh biscuits, rolls, muffins

  • Frozen fish and seafood

  • Frozen fruits and vegetables

  • Gasoline (all types)

  • Gasoline, unleaded midgrade

  • Gasoline, unleaded premium

  • Gasoline, unleaded regular

  • Ham

  • Ham, excluding canned

  • Household furnishings and operations

  • Household furnishings and supplies

  • Ice cream and related products

  • Infants' and toddlers' apparel

  • Juices and nonalcoholic drinks

  • Major appliances

  • Men's and boys' apparel

  • Men’s apparel

  • Men’s suits, sports coats, and outerwear

  • Motor fuel

  • Motor vehicle insurance

  • Nonalcoholic beverages and beverage materials

  • Nonfrozen noncarbonated juices and drinks

  • Other bakery products

  • Other fats and oils including peanut butter

  • Other foods

  • Other intercity transportation

  • Other meats

  • Other pork including roasts, steaks, and ribs

  • Other poultry including turkey

  • Pork

  • Processed fish and seafood

  • Professional services

  • Public transportation

  • Rice, pasta, cornmeal

  • Salt and other seasonings and spices

  • Snacks

  • Spices, seasonings, condiments, sauces

  • Stationery, stationery supplies, gift wrap

  • Uncooked beef roasts

  • Uncooked beef steaks

  • Used cars and trucks

  • Utilities and public transportation

  • Utility (piped) gas service

  • Women’s footwear

  • Women’s underwear, nightwear, swimwear and accessories

For each eligible series that was adjusted using X-13ARIMA-SEATS intervention analysis seasonal adjustment, a list of level shifts is provided in the file below, along with the identified causes (events). Outliers are also included in the file.

  • CPI Series subject to intervention analysis seasonal adjustment, 2023 (XLSX)

For additional information on seasonal adjustment in the CPI, please contact us at (202) 691-7000 or send an e-mail to cpi_info@bls.gov.

Last Modified Date: February 10, 2023