Consumer Price Index, Bureau of Labor Statistics, February 2016 

Intervention Analysis in Seasonal Adjustment 

In some Consumer Price Index (CPI) series, a process known as seasonal adjustment is used to identify 

and factor out seasonal movements. The resulting seasonally adjusted data reflect an estimate of 

nonseasonal price movements. The CPI uses the Bureau of the Census X-13ARIMA-SEATS software to 

calculate factors for seasonal adjustment of both historical and current series. 

Seasonal adjustment for some CPI series uses a technique called intervention analysis that is included in 

the Census X-13 seasonal adjustment program. Intervention analysis seasonal adjustment allows economic 

phenomena that are not seasonal in nature, such as outliers and level shifts, to be factored out of indexes 

before calculation of seasonal adjustment 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. Intervention analysis seasonal 

adjustment also makes it possible to account for seasonal shifts, resulting in a better seasonal adjustment in 

the periods before and after the shift occurred. For those CPI series adjusted using intervention analysis 

seasonal adjustment techniques, the resulting seasonal factors more accurately represent the underlying 

seasonal pattern. 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. 

When X-13ARIMA-SEATS is used to perform intervention analysis seasonal adjustment, unusual 

events are modeled as part of the seasonal adjustment process. 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 calculation of the seasonal factors. For a comprehensive discussion of 

intervention analysis seasonal adjustment, see "Improvements to CPI Procedures for Intervention Analysis 

Seasonal Adjustment" in the December 1996 issue of the CPI Detailed Report. 

BLS examined CPI series using the 8-year span from January 2008 through December 2015 with a few 

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

transportation, Airline fare, and Other lodging away from home including hotels and motels were adjusted 

using a 5-year data span from January 2011 through December 2015. 

A CPI series may receive intervention analysis seasonal adjustment (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. In January 2016, BLS adjusted the series listed 

below using intervention analysis seasonal adjustment techniques. 

Beverage materials including coffee and 

tea

Beef and veal 

Carbonated drinks 

Cereals and bakery products

Cheese and related products 

Cigarettes 

Coffee 

Dairy and related products 

Domestically produced farm food 

Eggs 

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 

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 

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 call Justin Yarros at 
(202) 691-5415, Samuel An at (202) 691-6970, or Marie Rogers at (202) 691-6984. They may also be 

reached by e-mail at yarros.justin@bls.gov, an.samuel@bls.gov, or rogers.marie@bls.gov. 

Consumer Price Index series – intervention analysis seasonal adjustment 

 

Level shift 

 Series period(s) Event Outliers 

Beef and veal 4/08

12/10-1/11

2/14

8/14

Increase in production 

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

2/11-7/11

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

2/08, 10/08, 

12/10

Carbonated drinks 8/08-10/08

5/11-8/11

Higher commodity costs combined with a declining

 demand for soft drinks

Higher costs of sweeteners, such as corn syrup and 

 other raw materials, were passed to consumers

4/08, 3/09, 

6/10, 10/10, 

12/10, 4/13

Cereals and bakery 

products

1/08-7/08

1/11-10/11

World grain shortage, rising ingredient costs, and 

 rising fuel costs led to higher prices

Southern U.S. and Mexico drought

None

Cheese and related 

products

4/08-7/08

8/08-8/09

2/11-9/11

1/14-5/14

Significantly greater demand for corn used in 

ethanol led to higher feed prices; higher energy 

and milk prices

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

Cigarettes 2/09-4/09 Increase in federal excise tax coupled with 

manufacturer price increases

None

Coffee 2/08-4/08

7/10-9/10

2/11-7/11

Speculation of a smaller Brazilian crop and a weak

 U.S. dollar

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
CPI series – intervention analysis seasonal adjustment-continued 

 

Level shift 

 Series period(s) Event Outliers 

Dairy and related products 3/08-7/08

1/09-8/09

2/11-9/11

10/13-5/14

Significantly greater demand for corn used in 

 ethanol led to higher feed prices; higher energy, 

 milk and cheese prices

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

3/08-9/08

2/11-9/11

1/14-5/14

Dry conditions in the Southern Plains and 

 Southeast; world grain shortage; increase in 

 global demand for more resource intensive food 

 and increase in price of fertilizer

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

Eggs 6/15 Shortages caused by the outbreak of the Avian 

 influenza A (H7N9) virus put sharp upward 

 pressure on wholesale and retail prices

None

Fresh milk other than 

whole

6/08-7/08

11/08-8/09

2/11-5/11

11/13-5/14

Significantly greater demand for corn used in 

 ethanol; higher energy and milk prices

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 6/08-7/08

11/08-8/09

2/11-5/11

11/13-5/14

Significantly greater demand for corn used in 

 ethanol; higher energy and milk prices

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
CPI series – intervention analysis seasonal adjustment-continued

 

Level shift 

 Series period(s) Event Outliers 

Fuel oil and other fuels 3/08-7/08

8/08-12/08

3/14

11/14-1/15

Record high crude oil prices 

Response in crude oil markets to world-wide

 economic downturn

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 9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Gasoline, unleaded 

midgrade

9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Gasoline, unleaded 

premium

9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Gasoline, unleaded regular 9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

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

9/08

10/09

6/11-7/11

Manufacturers raising prices due to higher

commodity costs and declining U.S. sales

Forecasted increase of orange crop

Expected smaller than usual crop due to winter 

freeze, diseases and a drought in Florida

2/08, 4/08, 

3/09, 10/10, 

12/10, 9/12, 

5/13, 3/15
CPI series – intervention analysis seasonal adjustment-continued

 

Level shift 

 Series period(s) Event Outliers 

Leased cars and trucks 7/08-10/08

12/08-2/09

6/09

2/10

10/15-12/15

Decreased demand for SUVs and light trucks due to 

 higher gasoline prices, leading to lower residual 

 prices and higher lease prices

Decreased demand for SUVs and light trucks due to 

higher gasoline prices, leading to lower residual 

prices and higher lease prices

“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 

Recalls put downward pressure on residual values 

 for used cars, resulting in higher lease prices

Lower residual values pushed lease prices higher

08/09 

Motor fuel 9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

New cars 3/11-6/11 Pent-up demand from earlier delayed purchases due 

to declining economic conditions resulting in 

higher current demand; significantly fewer 

discounts and incentives; shortages in supply 

due to earthquake and tsunami in Japan leading 

to below average inventories

 

None

Nonalcoholic beverages 

and beverage materials

8/08-10/08

2/11-7/11

Higher commodity costs combined with a declining

 demand for soft drinks

Bad weather conditions in Brazil and Colombia 

 and increased supply hoarding for coffee;

 higher soft drink prices 

4/08, 3/09, 

12/10

Nondurables 9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Nondurables less food 9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers

Nondurables less food and 

apparel

9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

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

9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

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

9/08-12/08

6/09

11/14-1/15

Response in crude oil markets to world-wide

 economic downturn

Return to normal pricing

Weak global demand along with increased supply

 from continued growth in North American crude 

 production

None

Other households fuels 4/08-7/08

12/13-2/14

2/14-4/14

Record high crude oil prices 

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

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

physician practices 

2/15, 11/15

Pork 4/08

4/14

Rising corn (feed) prices; higher exports that 

 diverted pork available for domestic consumption

PED virus led to millions of piglets and slaughterready pigs being killed

None

Prescription drugs 10/12-12/12

5/13

12/13

Expiration of patents resulted in increased 

availability of generic alternatives

Higher concentration than normal of brand to 

 generic conversions due to patent loss for a 

 variety of drugs

Higher concentration than normal of brand to 

 generic conversions due to patent loss for a

 variety of drugs

2/14

Public transportation None 3/13

Tobacco and smoking 

products

2/09-4/09 Increase in federal excise tax coupled with 

 manufacturer price increases

None
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers

Utilities and public 

transportation

2/08-7/08

8/08-11/08

1/09-4/09

12/13-3/14

Effect of a widening storage deficit, combined with

 strong summer demand

Introduction of lower winter gas rates, and recovery

 of storage inventories

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 2/08-7/08

8/08-9/08

3/09-5/09

1/14-3/14

Effect of a widening storage deficit, combined with

 strong summer demand

Introduction of lower winter gas rates and recovery

 of storage inventories

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

None