Consumer Price Index, Bureau of Labor Statistics, February 2017

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 2009 through December 2016 

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

Public transportation, and Airline fare were adjusted using a 5-year data span from January 2012 through 

December 2016. 

In January 2017, BLS adjusted the series listed below using IASA techniques. 

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 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

Other lodging away from home including 

 hotels and motels

Physicians’ services

Pork

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 

Beef and veal 12/10-1/11

2/14

8/14

Higher input costs (wheat and corn) due to drought

Smallest herd size since 1951; result of years of 

 drought and high feed prices

Higher feed prices coupled with increased demand

7/09

Beverage materials 

including coffee and tea

3/09-4/09

8/10-9/10

2/11-5/11

Declining and volatile tea industry prices due to 

 global recession; coffee prices fell on concern 

 that prolonged recession would reduce demand 

 for raw materials

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

Carbonated drinks 5/11-8/11 Higher costs of sweeteners, such as corn syrup and 

 other raw materials, were passed to consumers

3/09, 12/10, 

4/13, 4/16

Cereals and bakery 

products

1/11-10/11 Southern U.S. and Mexico drought None

Cigarettes 5/10

7/14

Prices adjust to local and national excise taxes

Drop in cigarette prices as inventory cleared 

nationwide and especially in the Midwest

None

Coffee 7/10-9/10

2/11-7/11

Bad weather conditions in Brazil and Colombia, 

planned supply hoarding by exporters Brazil and 

Vietnam, and a general manufacturer’s price 

increase 

Bad weather conditions in Brazil and Colombia and

 increased supply hoarding

12/10

Dairy and related products 1/09-8/09

2/11-9/11

10/13-5/14

Decline in domestic demand and softer export sales 

 due to exchange rates; increased milk 

 production led to larger cheese stocks

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

None

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

None
CPI series – intervention analysis seasonal adjustment-continued

 

Level shift

 Series period(s) Event Outliers

Fresh milk other than 

whole

01/09-8/09

2/11-5/11

11/13-5/14

Decline in domestic demand and softer export sales 

 due to exchange rates; increased milk 

 production

Higher feed prices (corn/soybean), rising energy 

 prices, and strong international demand for dairy 

 products

Slower increase in milk production; strong domestic 

 and foreign demand

None

Fresh whole milk 1/09-3/09

2/11-5/11

11/13-5/14

Decline in domestic demand and softer export sales 

 due to exchange rates; increased milk 

 production

Higher feed prices (corn/soybean), rising energy 

 prices, and strong international demand for dairy 

 products 

Slower increase in milk production; strong domestic 

 and foreign demand

None

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

None

Gasoline, all types 6/09

11/14-1/15

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Gasoline, unleaded 

midgrade

6/09

11/14-1/15

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Gasoline, unleaded 

premium

6/09

11/14-1/15

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Gasoline, unleaded regular 6/09

11/14-1/15

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production 

None

Ice cream and related 

products

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

ingredients used in ice cream

5/09, 2/12, 

12/14

Juices and nonalcoholic

drinks 

6/11-7/11 Expected smaller than usual crop due to winter 

freeze, diseases and a drought in Florida

3/09, 10/10, 

12/10, 9/12, 

5/13
CPI series – intervention analysis seasonal adjustment-continued

 

Level shift

 Series period(s) Event Outliers

Leased cars and trucks 6/09

10/15-12/15

“Car Allowance Rebate System” initiative led to a

 lower supply of used cars and an increase in

 used car prices, allowing manufacturers to

 decrease leased car prices as the residual prices

 returned to normal 

Lower residual values pushed lease prices higher

Motor fuel 6/09

11/14-1/15

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Nonalcoholic beverages 

and beverage materials

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

 and increased supply hoarding for coffee; 

 higher soft drink prices 

3/09, 12/10

Nondurables 6/09

11/14-1/15

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Nondurables less food 6/09

11/14-1/15

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Nondurables less food and 

apparel

6/09

11/14-1/15

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Nondurables less food and 

beverages

6/09

11/14-1/15

Return to normal pricing 

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None 

Nondurables less food, 

beverages, and apparel

6/09

11/14-1/15

Return to normal pricing 

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

Other lodging away from 

home including hotels and 

motels

None 6/15
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

Series period(s) Event Outliers

Physicians’ services 5/12-9/12 Higher prices mostly due to hospitals buying 

physician practices 

2/15

11/15 Cash price increases for office visits

4/16-8/16 Large increase in insurance reimbursements due to 

contract updates between insurance companies 

and providers

Pork 4/14 PED virus led to millions of piglets and slaughterready pigs being killedNone

12/14-4/15 Recovery of pork supply after PED virus event

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

Tobacco and smoking 

products

5/10

7/14

Prices adjust to local and national excise taxes

Drop in cigarette prices as inventory cleared 

 nationwide and especially in the Midwest

None

Utilities and public 

transportation

1/09-4/09

12/13-3/14

Increased inventories compared to 12 months prior, 

 weak demand and reduced spot market prices

Dramatic increase in demand due to extreme winter 

 in Midwest and Northeast 

4/14

Utility (piped) gas service 3/09-5/09

1/14-3/14

Increased inventories compared to 12 months prior;

 weak demand and reduced spot market prices

Dramatic increase in demand due to extreme winter 

 in Midwest and Northeast

11/09