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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 2018 through December 2025 with some exceptions. The following series were examined using a 7-year span from January 2019 through December 2025: Used cars and trucks. The following series were examined using a 6-year span from January 2020 through December 2025: Pet services and Olives, pickles, relishes.

Due to the lapse in funding, BLS could not publish October 2025 indexes most CPI series. This resulted in BLS using atypical methods in order to maintain a continuous time series on which to calculate revises seasonal factors, including a seasonal factor for October 2025. For more information, see “How are the missing October 2025 data being handled in the seasonal adjustment process?” on the Seasonal Adjustment questions and answers page.

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 forward seasonal factors for 2026 for New Vehicles, New cars and New trucks were determined using eight 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 2024, the Consumer Price Index program updated the mileage adjustment for the used cars and trucks index. Historically, a single, stable mileage amount was estimated and applied to each sampled vehicle and remained unchanged throughout the year. The mileage adjustment was replaced with a monthly average mileage amount based on the age of the sampled used vehicle. The forward seasonal factors for 2026 were determined from seven years of estimated 1-month price change data that incorporates the updated mileage adjustment. The data used to determine seasonal factors for the series is available on the CPI seasonal adjustment web page. 

In January 2026, BLS adjusted the series listed below using IASA techniques:

  • Airline fares

  • Beef and veal

  • Cable, satellite, and live streaming television service

  • Car and truck rental

  • Carbonated drinks

  • Cereals and bakery products

  • Chicken

  • Club membership for shopping clubs, fraternal, or other organizations, or participant sports fees

  • Eggs

  • Electricity

  • Elementary and high school tuition and fees

  • Frankfurters

  • Fuel oil

  • Fuel oil and other fuels

  • Gasoline (all types)

  • Gasoline, unleaded midgrade

  • Gasoline, unleaded premium

  • Gasoline, unleaded regular

  • Ham

  • Housing at school, excluding board

  • 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

  • Nondurables

  • Nondurables less food

  • Nondurables less food and apparel

  • Nondurables less food and beverages

  • Nondurables less food, beverages, and apparel

  • Nonfrozen noncarbonated juices and drinks

  • Olives, pickles, relishes

  • Other bakery products

  • Other intercity transportation

  • Other lodging away from home including hotels and motels

  • Personal care

  • Pet services

  • Pet services including veterinary

  • Pork

  • Poultry

  • Public transportation

  • Rice, pasta, cornmeal

  • Sugar and sweets

  • Technical and business school tuition and fees

  • Tools, hardware and supplies

  • Uncooked beef roasts

  • Uncooked beef steaks

  • Uncooked ground beef

  • Used cars and trucks

  • Utilities and public transportation

  • Utility (piped) gas service

  • 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 on the CPI seasonal adjustment web page, along with the identified causes (events). Outliers are also included in the file.

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 19, 2026