Consumer Price Index, Bureau of Labor Statistics, February 2018 

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, 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 2010 through December 2017 

with some exceptions. New vehicles, New cars and trucks, New Cars, Used cars and trucks, Public 

transportation, and Airline fare were adjusted using a 5-year data span from January 2013 through 

December 2017. Fuel oil was seasonally adjusted for the first time in the five year revision span.

In January 2018, BLS adjusted the series listed below using IASA techniques. 

Airline Fare 

Beef and veal 

Beverage materials including coffee and 

tea 

Carbonated drinks 

Cereals and bakery products 

Cigarettes 

Coffee 

Dairy and related products 

Domestically produced farm food 

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 

Ice cream and related products 

Juices and nonalcoholic drinks 

Leased cars and trucks 

Motor fuel 

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 

Public transportation 

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 

Airline fare

Beef and veal

None

12/10-1/11

8/14

6/17

Higher input costs (wheat and corn) due to drought

Higher feed prices coupled with increased demand

Restrictions were lifted and as a result the U.S. were 

able to export to China for first time since 2003

3/13, 5/13, 

6/13

Beverage materials 

including coffee and tea

8/10-9/10

2/11-6/11

Bad weather conditions in Brazil and Colombia,

 planned supply hoarding by exporters in Brazil

 and Vietnam, and a general manufacturer’s 

 price increase for coffee

Bad weather conditions in Brazil and Colombia and 

increased supply hoarding for coffee

12/10, 8/17

Carbonated drinks 4/11-7/11 Higher costs of sweeteners, such as corn syrup and 

 other raw materials, were passed to consumers

10/10, 12/10, 

2/11, 4/16

Cereals and bakery 

products

1/11-10/11 Southern U.S. and Mexico drought 10/15

Cigarettes 5/10

7/14

4/17

Prices adjust to local and national excise taxes

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

Coffee 8/10-9/10

2/11-7/11

Bad weather conditions in Brazil and Colombia,

 planned supply hoarding by exporters in Brazil

 and Vietnam, and a general manufacturer’s 

 price increase for coffee

Bad weather conditions in Brazil and Colombia and 

increased supply hoarding for coffee

12/10, 8/17

Dairy and related products 2/11-9/11

10/13-5/14

Higher feed prices, rising energy prices, and strong 

 international demand for dairy products

Increase in cheese prices due to higher demand and 

 stronger export sales

12/14

Domestically produced 

farm food

3/11

1/14-5/14

Southern U.S. and Mexico drought; global demand 

for biofuel increased demand for feedstock crop 

(i.e. production of corn-based ethanol)

Higher beef and veal prices as a result of drought 

 (i.e. Texas and California) and higher feed 

 prices; higher diary prices

12/14
CPI series – intervention analysis seasonal adjustment-continued 

 

Level shift 

 Series period(s) Event Outliers 

Fresh milk other than 

whole

2/11-5/11

11/13-5/14

Higher feed prices (corn/soybean), rising energy 

 prices, and strong international demand for dairy 

 products

Price of corn was low. Strong domestic and foreign 

demand

None

Fresh whole milk 2/11-5/11

11/13-5/14

Higher feed prices (corn/soybean), rising energy 

 prices, and strong international demand for dairy 

 products 

Price of corn was low. Strong domestic and foreign 

demand

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

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

Ice cream and related 

products

11/10-2/11 Strong foreign demand for whey, butter fat and other

ingredients used in ice cream

None

Juices and nonalcoholic

drinks

None 12/10, 9/12
CPI series – intervention analysis seasonal adjustment-continued

 

Level shift 

 Series period(s) Event Outliers 

Leased cars and trucks 10/15-12/15

2/17

9/17-11/17

Lower residual values pushed lease prices higher

Lower residual values and higher rent charges

Lower residual values and higher rent charges

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

Nonalcoholic beverages 

and beverage materials

2/11-7/11 Bad weather conditions in Brazil and Colombia 

 and increased supply hoarding for coffee;

 higher prices for sweeteners used in soft drinks

12/10

Nondurables 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

Nondurables less food 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

Nondurables less food and 

apparel

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

Nondurables less food and 

beverages

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 

Nondurables less food, 

beverages, and apparel

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

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

Physician’s 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
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers

Prescription drugs 10/12-12/12

5/16-8/16

Expiration of patents resulted in increased 

availability of generic alternatives

Slowdown in blockbuster drugs and increase in 

 mergers and acquisitions led to lower 

 competition, pushing drug prices higher

12/13

Public transportation None 3/13, 5/13

6/13

Tobacco and smoking 

products

5/10

7/14

4/17

Prices adjust to local and national excise taxes

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

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

 in Midwest and Northeast

None