Consumer Price Index, Bureau of Labor Statistics, February 2015 

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. 

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 2015, BLS adjusted the series listed 

below using intervention analysis seasonal adjustment techniques. BLS examined these series using the 8-

year span from January 2007 through December 2014, with the exception of Used cars and trucks, which 

was adjusted using data from January 2010 through December 2014. 

Airline fare 

Bakery products 

Beverage materials including coffee and 

tea 

Carbonated drinks 

Cereals and bakery products 

Cheese and related products 

Cigarettes 

Coffee 

Electricity 

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 

New cars and trucks 

New 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 

Prescription drugs 

Public transportation 

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 call Carlyle Jackson 

at (202) 691-6984 or Christopher Graci at (202) 691-5826. They may also be reached at 

Jackson.Carlyle@bls.gov or Graci.Christopher@bls.gov by e-mail. 
Consumer Price Index series – intervention analysis seasonal adjustment 

 

 Level shift 

 Series period(s) Event Outliers 

Airline fare 4/08-7/08 

8/08-1/09 

8/09-12/09 

11/13

(Seasonal Shift) 

Higher fuel surcharges implemented by airlines due 

 to increased fuel costs 

Return to normal pricing

Reduction in airplane capacity led to less availability, 

 fewer discounts, and higher prices 

A higher percentage of web-based airline fares were 

 included in the sample, better capturing advance 

 prices for holiday travel 

3/13 

Bakery products 

Beverage materials 

including coffee and tea 

1/08-5/08 

9/10

2/11-8/11 

World grain shortage, rising ingredient costs, and 

rising fuel costs led to higher prices

Bad weather conditions in Brazil and Colombia, 

 planned supply hoarding by exporters in Brazil 

 and Vietnam, and a general manufacturer’s 

 price increase 

Bad weather conditions in Brazil and Colombia and 

increased supply hoarding 

None 

2/08

Carbonated drinks 

Cereals and bakery 

products

Cheese and related 

products

8/08-10/08 

1/08-5/08 

6/07-10/07 

4/08-8/08 

12/08-05/09 

1/11

3/11-9/11 

2/14-5/14 

Higher commodity costs combined with a declining 

 demand for soft drinks 

World grain shortage, rising ingredient costs, and 

rising fuel costs led to higher prices

Cheese prices increased due to volatile spot trading, 

strong demand and higher Class III milk prices 

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 

Increased milk output and ample 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 

3/07, 12/07, 

4/08, 3/09, 

10/10, 12/10 

None

None

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

manufacturer price increases 

None

Coffee 4/08 

7/10-9/10 

2/11-8/11 

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 during the modeled period 

Bad weather conditions in Brazil and Colombia and 

increased supply hoarding 

None 
CPI series – intervention analysis seasonal adjustment-continued 

 

 Level shift 

 Series period(s) Event Outliers 

Electricity 

Gasoline, all types

None 

9/08-12/08 

6/09

10/14-12/14 

None 

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 

2/10, 4/14 

None

Gasoline, unleaded 

midgrade

Gasoline, unleaded 

premium

9/08-12/08 

6/09

10/14-12/14 

9/08-12/08 

6/09

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 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing

None 

None

Gasoline, unleaded regular 9/08-12/08 

6/09

10/14-12/14 

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 

None 

Juices and nonalcoholic 

drinks 

Leased cars and trucks 

9/08 

7/08-10/08 

12/08-2/09 

6/09

2/10

Higher commodity costs combined with a declining 

 demand for soft drinks 

Decreased demand for SUV’s and light trucks due to 

higher gasoline prices, leading to lower residual 

prices and higher lease prices 

Decreased demand for SUV’s 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 

3/07, 12/07, 

4/08, 3/09, 

12/10 

08/09 
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers 

Motor fuel 

New cars 

New cars and trucks

New trucks 

New vehicles

Nonalcoholic beverages 

and beverage materials 

9/08-12/08 

6/09

10/14-12/14 

9/09-11/09 

3/11-6/11 

2/09-7/09 

9/09-11/09 

3/11-6/11 

1/09-7/09 

9/09-11/09 

2/09-7/09 

9/09-11/09 

3/11-6/11 

8/08-10/08 

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 

Production cuts by manufacturers in spring 2009 

resulting in all-time low levels of inventories 

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 

Return to regular pricing was more pronounced due 

to heavy losses from deep discounting in the 

summer and fall of 2008; dramatic production 

cuts spurred by high inventories 

Production cuts by manufacturers in spring 2009 

resulting in all-time low levels of inventories 

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 

Return to regular pricing was more pronounced due 

 to heavy losses from deep discounting in the 

 summer and fall of 2008; dramatic production 

 cuts spurred by high inventories 

Production cuts by manufacturers in spring 2009

 resulting in all-time low levels of inventories 

Return to regular pricing was more pronounced due 

to heavy losses from deep discounting in the 

summer and fall of 2008; dramatic production 

cuts spurred by high inventories 

Production cuts by manufacturers in spring 2009 

resulting in all-time low levels of inventories 

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 

Higher commodity costs combined with a declining 

 demand for soft drinks 

None 

None

None 

None

None

3/07, 12/07, 

4/08, 3/09, 

12/10
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers 

Nondurables 9/08-12/08 

6/09 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

None 

Nondurables less food 9/08-12/08 

6/09

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing

None 

Nondurables less food and 

apparel

Nondurables less food and 

beverages

Nondurables less food, 

beverages, and apparel 

Prescription drugs

Public transportation

Tobacco and smoking 

products

9/08-12/08 

6/09

9/08-12/08 

6/09

9/08-12/08 

6/09

1/07-3/07 

10/12-12/12 

5/13

12/13

4/08-7/08 

8/08-1/09 

8/09-12/09 

11/13

(Seasonal Shift) 

2/09-4/09 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

Significantly lower prices for some generic drugs due 

to patent loss and new generic drug discount 

program implementations at several major 

retailers 

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 

Higher fuel surcharges implemented by airlines due 

 to increased fuel costs 

Return to normal pricing

Reduction in airplane capacity led to less availability, 

fewer discounts, and higher prices 

A higher percentage of web-based airline fares were 

included in the sample, better capturing advance 

prices for holiday travel 

Increase in federal excise tax coupled with 

manufacturer price increases 

None 

None 

None

2/14

3/13

None
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers 

Used Cars and Trucks 

Utilities and public 

transportation

Utility (piped) gas service 

3/11-6/11 

2/08-7/08 

8/08-1/09 

12/13-3/14 

3/08-7/08 

1/14-3/14 

Reduced supply of new cars in 2009 and 2010 due 

to the recession led to lower supply of used cars 

in 2011; shortages caused by the earthquake in 

Japan affected parts and supplies resulting in 

increased new car prices and higher demand for 

used cars 

Effect of a widening storage deficit, combined with

 strong summer demand 

Introduction of lower winter gas rates, and recovery 

 of storage inventories 

Dramatic increase in demand due to extreme winter 

in Midwest and Northeast 

Effect of a widening storage deficit, combined with

 strong summer demand 

Dramatic increase in demand due to extreme winter 

in Midwest and Northeast 

None 

4/14

None