Consumer Price Index, Bureau of Labor Statistics, February 2020

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 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. 

A CPI series may receive IASA if the series is directly adjusted and has a relative importance greater 

than 0.5% to the U.S. city average All items index. If IASA is used for a component of the seasonally 

adjusted U.S. city average All items index, series which are subsets of the component series are also 

eligible for IASA. 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 2012 through December 2019

with some exceptions. New vehicles, New cars and trucks, and New trucks were examined with a 5-year 

span from January 2015 through December 2019. Additionally, Airline fare, Public transportation, and 

Utilities and public transportation were adjusted using a 6-year span from January 2014 through December 

2019. In 2018, the Used cars and trucks index changed its estimation methodology from using a 3-month 

moving average of depreciated prices to a 1-month price change approach. Seasonal factors for this series 

were determined using five years of estimated 1-month price change. 

In January 2020, BLS adjusted the series listed below using IASA techniques. 

Beverage materials including coffee and 

tea

Carbonated drinks

Cereals and bakery products

Cheese and related products

Cigarettes

Coffee

Dairy and related products

Fresh milk other than whole

Fresh whole milk

Fuel oil 

Fuel oil and other fuels

Gasoline, all types 

Gasoline, unleaded midgrade 

Gasoline, unleaded premium

Gasoline, unleaded regular

Juices and nonalcoholic drinks

Milk

Motor fuel 

New cars

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

Other household fuels

Physicians’ services

Professional services

Propane, kerosene, and firewood

Tobacco and smoking products

Utilities and public transportation

Utility (piped) gas service 

For each series that was adjusted using X-13ARIMA-SEATS intervention analysis seasonal adjustment, 

a list of level shifts is provided in the table below, along with the identified causes (events). Outliers are 

also included in the list. 
For further information, write to the Bureau of Labor Statistics, Division of Consumer Prices and Price 

Indexes, 2 Massachusetts Avenue, NE, Room 3615, Washington, DC 20212-0001, or send an e-mail to

cpiseas@bls.gov. 

Consumer Price Index series – intervention analysis seasonal adjustment

 

Level shift

 Series period(s) Event Outliers 

Beverage materials 

including coffee and tea

2/17 High robusta coffee prices due to weather in the two 

largest robusta producing countries, Vietnam 

and Brazil. Vietnam experienced heavy rains 

and floods during their harvesting season in late 

2016 which affected supplies. Brazil’s robusta 

production was reduced due to above average 

temp and prolonged dry spells

12/17

Carbonated drinks 7/18-10/18 Broad based cost increases and new tariffs on 

 aluminum and steel imported from the E.U., 

 Canada, and Mexico

4/16

Cereals and bakery 

products

None 10/15

Cheese and related

products

Cigarettes

10/13-5/14

7/14

4/17

Increase in cheese prices due to higher demand and 

 stronger export sales

Drop in cigarette prices as inventory cleared 

nationwide and especially in the Midwest

New California excise tax, along with major 

manufacturer’s price increases

None

None

Coffee 2/17 High robusta coffee prices due to weather in the two 

largest robusta producing countries, Vietnam and 

Brazil. Vietnam experienced heavy rains and 

floods during their harvesting season in late 2016 

which affected supplies. Brazil’s robusta 

production was reduced due to above average 

temp and prolonged dry spells 

12/17

Dairy and related products 10/13-5/14 Increase in cheese prices due to higher demand and 

 stronger export sales

None

Fresh milk other than 

whole

11/13-5/14

2/15

Price of corn was low. Strong domestic and foreign 

demand

Strong U.S. dollar and CA port strike affected 

 exports. Wholesale prices also fell because of 

 strong milk production

None

Fuel oil 11/14-1/15 Low prices due to increased oil production and 

 reduced non-domestic demand

3/15

Fuel oil and other fuels 3/14

11/14-1/15

High propane prices caused by unseasonably low 

 temperatures and high farm demand, driving 

 residential supply down

Low prices due to increased oil production and 

 reduced non-domestic demand

3/15
CPI series – intervention analysis seasonal adjustment-continued

 

Level shift

 Series period(s) Event Outliers

Fuel oil and other fuels 3/14

11/14-1/15

High propane prices caused by unseasonably low 

 temperatures and high farm demand, driving 

 residential supply down

Low prices due to increased oil production and 

 reduced non-domestic demand

3/15

Gasoline, all types 11/14-1/15

9/15

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

Low prices due to global oil glut

2/16

Gasoline, unleaded 

midgrade

11/14-1/15

9/15

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

Low prices due to global oil glut

2/16

Gasoline, unleaded 

premium

11/14-1/15

9/15

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

Low prices due to global oil glut

2/16

Gasoline, unleaded regular 11/14-1/15

9/15

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

Low prices due to global oil glut

2/16

Juices and nonalcoholic

drinks 

7/18-10/18

12/18-2/19

Price increase due to new tariffs on aluminum and 

 steel imports. Also, Hurricane Irma in 2017 

 reduced Florida orange crops into 2018. Florida 

 is main supplier of oranges used for juice, so 

 declining crop yields forced commodity prices 

 higher in 2018 

Price increase due to new tariffs on aluminum and 

 steel imports. Also, Hurricane Irma in 2017 

 reduced Florida orange crops into 2018. Florida 

 is main supplier of oranges used for juice, so 

 declining crop yields forced commodity prices 

 higher in 2018 

None

Milk 11/13-5/14

2/15

Price of corn was low. Strong domestic and foreign 

demand

Strong U.S. dollar and CA port strike affected 

 exports. Wholesale prices also fell because of 

 strong milk production

None

Motor fuel 11/14-1/15

9/15

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

Low prices due to global oil glut

2/16

New cars 7/17 Surplus supply and lack of consumer demand after 

 previous years’ record sales

1/17, 4/18
CPI series – intervention analysis seasonal adjustment-continued

 

Level shift

 Series period(s) Event Outliers

Nonalcoholic beverages 

and beverage materials

12/18-2/19 Price increase due to new tariffs on aluminum and 

 steel imports. Also, Hurricane Irma in 2017 

 reduced Florida orange crops into 2018. Florida 

 is main supplier of oranges used for juice, so 

 declining crop yields forced commodity prices 

 higher in 2018 

2/17

Nondurables 11/14-1/15 Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Nondurables less food 11/14-1/15 Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Nondurables less food and 

apparel

11/14-1/15 Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Nondurables less food and 

beverages

11/14-1/15 Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None 

Nondurables less food, 

beverages, and apparel

11/14-1/15 Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Other households fuels 12/13-2/14

2/14-4/14

High propane prices caused by unseasonably low 

 temperatures and high farm demand, driving 

 residential supply down

Return to normal pricing and supply recovery after 

 low temperatures

None

Physicians’ services 11/15

4/16-8/16

4/17

Cash price increases for office visits

Large increase in insurance reimbursements due to 

contract updates between insurance companies 

and providers

Large cash price decreases for office visits

2/15

Professional services 11/15

4/16-8/16

4/17

Cash price increases for office visits

Large increase in insurance reimbursements due to 

 contract updates between insurance companies 

 providers

Large cash price decreases for office visits

None

Propane, kerosene, and 

firewood

None 1/14, 2/14, 

3/14

Tobacco and smoking 

products

7/14

4/17

Drop in cigarette prices as inventory cleared 

 nationwide and especially in the Midwest

New California excise tax, along with major 

 manufacturer’s price increases

None
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

Series period(s) Event Outliers

Utilities and public 

transportation

None 3/14

Utility (piped) gas service 3/13-5/13

12/13-3/14

Cold winter temperatures and large withdrawals of 

 natural gas storage inventories

Dramatic increase in demand due to extreme winter 

 in Midwest and Northeast

2/18