Intervention Analysis in Seasonal Adjustment, 2024 : U.S. Bureau of Labor Statistics

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

Intervention Analysis in Seasonal Adjustment, 2024

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 2016 through December 2023 with some exceptions. The following series were examined using a 6-year span from January 2018 through December 2023: 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 2024 seasonal adjustment factors for New Vehicles, New cars and New trucks were determined using six 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 will be replaced with a monthly average mileage amount based on the age of the sampled used vehicle. The forward seasonal factors for 2024 were determined from 5 years of estimated 1-month price change data that incorporates the updated mileage adjustment. The data used to determine forward seasonal factors for 2024 for this series are available on the CPI seasonal adjustment web page.

In January 2024, BLS adjusted the series listed below using IASA techniques:

Airline fares
Appliances
Beef and veal
Boy’s apparel
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
Fuel oil
Gasoline (all types)
Gasoline, unleaded midgrade
Gasoline, unleaded premium
Gasoline, unleaded regular
Ham
Ham, excluding canned
Ice cream and related products
Infants' and toddlers' apparel
Juices and nonalcoholic drinks
Major appliances
Medical care services
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
Nursing homes and adult daycare
Other bakery products
Other intercity transportation
Other meats
Other miscellaneous foods
Other poultry including turkey
Pork
Professional services
Public transportation
Rice, pasta, cornmeal
Uncooked beef roasts
Uncooked beef steaks
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, along with the identified causes (events). Outliers are also included in the file.

CPI Series subject to intervention analysis seasonal adjustment, 2024 (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 9, 2024