Consumer Price Index, Bureau of Labor Statistics, February 2019

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 nonseasonal 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, s uch 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, s uch 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 2011 through December 2018

with some exceptions. Allofthe food related series below were examined with a 7-year span, in order to 

mitigate the impact on seasonal adjustment of a U.S. – South American drought in 2011 that pushed food 

prices higher throughout the year. Additionally, New vehicles,New cars and trucks, New cars, Public 

transportation, and Airline fare were adjusted using a 5-year span from January 2014 through December 

2018.

In January 2019, BLS adjusted the series listed below using IASAtechniques. 

Beverage materials including coffee and 

tea

Carbonated drinks

Cereals and bakery 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

Medical care commodities

Medical care services

Medicinal drugs

Milk

Motor fuel 

New cars

New cars and trucks

New vehicles

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

Prescription drugs

Professional services

Propane, kerosene, and firewood

Tobacco and smoking products

Used cars and trucks

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

None 2/17

Carbonated drinks None 4/16

Cereals and bakery 

products

None 10/15

Cigarettes 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’sprice increases

None

Coffee None 2/17

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

 stronger export sales

12/14

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

Fresh whole 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

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

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

None
CPI series – intervention analysis seasonal adjustment-continued

Level shift

 Series period(s) Event Outliers

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

None

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

None

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

None

Juices and nonalcoholic

drinks

None 9/12, 2/17, 

12/17

Medical care commodities 5/16-8/16 Slowdown in blockbuster drugs and increase in 

 mergers and acquisitions leading to lower 

 competition pushed drug prices higher

None

Medical care services 8/16 Large increase in insurance reimbursements due to 

contract updates between health insurance 

companies and providers

None

Medicinal drugs 5/16-8/16 Slowdown in blockbuster drugs and increase in 

 mergers and acquisitions leading to lower 

 competition pushed drug prices higher

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

None

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

 previous years’ record sales

1/17, 4/18

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

 previous years’ record sales

1/17, 4/18

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

 previous years’ record sales

1/17, 4/18

Nonalcoholic beverages 

and beverage materials

None 2/17, 5/17
CPI series – intervention analysis seasonal adjustment-continued

Level shift

 Series period(s) Event Outliers

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

Prescription drugs 5/16-8/16 Slowdown in blockbuster drugs and increase in 

 mergers and acquisitions leading to lower 

 competition pushed drug prices higher

None

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’sprice increases

None

Used cars and trucks 9/18-11/18 Increase in demand due to depleted used car and

 truck market after natural disasters

8/18
CPI series – intervention analysis seasonal adjustment-continued

 Level shift

Series period(s) Event Outliers

Utilities and public 

transportation

12/13-3/14 Dramatic increase in demand due to extreme winter 

 in Midwest and Northeast

4/14, 2/17

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

12/18