Consumer Price Index, Bureau of Labor Statistics, February 2012 

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-12-ARIMA software to calculate 

factors for seasonal adjustment of both historical and current series. 

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

the Census X-12 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 intervention. As a 

result, level shifts and outliers, removed for the calculation of seasonal factors, are present in the seasonally 

adjusted series. 

When X-12-ARIMA is used to perform intervention analysis seasonal adjustment, unusual events are 

modeled as part of the seasonal adjustment process. X-12-ARIMA’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 X-12-ARIMA and 

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 2012, BLS adjusted the series listed 

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

year span from January 2004 through December 2011. 

Airline fare 

Beverage materials including coffee and 

tea 

Cable and satellite television and radio 

 service 

Carbonated drinks 

Cheese and related products 

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 

Other lodging away from home including 

 hotels and motels 

Prescription drugs 

Public transportation 

Used cars and trucks 

Utilities and public transportation 

Utility (piped) gas service 

For each series that was adjusted using X-12-ARIMA 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 David Levin at 

(202) 691-6968. Mr. Levin may also be reached at Levin.David@bls.gov by e-mail. 
Consumer Price Index series – intervention analysis seasonal adjustment 

 

 Level shift 

 Series period(s) Event Outliers 

Airline fare 2/08-7/08 

7/08-2/09 

8/09-12/09 

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. 

None 

 

Beverage materials 

including coffee and tea 

11/04-4/05 

9/10 

Smaller Brazilian coffee crop, rising Brazilian 

 Currency 

Bad weather conditions in Brazil and Colombia, 

2/08 

 

12/10-8/11 

 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 

Cable and satellite 

television and radio service 

None None 6/05, 7/05 

 

Carbonated drinks 

Cheese and related 

products 

8/08-10/08 

5/04 

9/05 

6/07-10/07 

4/08-8/08 

12/08-05/09 

1/11 

3/11-9/11 

Higher commodity costs combined with a declining 

 demand for soft drinks 

Very low spring stocks of cheese and butter, plus 

lower production of raw milk 

Response to tight cheese and dairy market 

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 

3/07, 12/07, 

4/08, 3/09, 

12/10 

4/06, 6/06 

 

Coffee 11/04-4/05 

4/08 

7/10-9/10 

12/10-8/11 

Smaller Brazilian coffee crop, rising Brazilian 

 currency 

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 

 

Electricity 1/06 Larger than normal increase due to recovery from 

 Hurricane Katrina 

11/05, 2/10 
CPI series – intervention analysis seasonal adjustment-continued 

 

 Level shift 

 Series period(s) Event Outliers 

Gasoline, all types 6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09 

Damage to U.S. oil refineries resulting from 

 Hurricane Katrina, along with record global 

 petroleum demand and continued political unrest 

 in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

 Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

None 

Gasoline, unleaded 

midgrade 

Gasoline, unleaded 

premium 

6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09 

6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09 

Damage to U.S. oil refineries resulting from 

 Hurricane Katrina, along with record global 

 petroleum demand and continued political unrest 

 in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

 Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

Damage to U.S. oil refineries resulting from 

 Hurricane Katrina, along with record global 

 petroleum demand and continued political unrest 

 in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

 Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

None 

None 

 

Gasoline, unleaded regular 6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09 

Damage to U.S. oil refineries resulting from 

 Hurricane Katrina, along with record global 

 petroleum demand and continued political unrest 

 in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

 Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

None 

 

Ice cream and related 

products 

6/09 

11/10-2/11 

Large decrease in ingredient prices during May 

Strong foreign demand for whey, butter fat and other 

ingredients used in ice cream 

3/05, 6/05, 

8/06 

 
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers 

Juices and nonalcoholic 

drinks 

2/05-4/05 

8/08-10/08 

Orange juice futures increased due to cuts in 

 projected citrus production of Florida, related to 

 the previous hurricane season 

Higher commodity costs combined with a declining 

 demand for soft drinks 

3/07, 12/07, 

4/08, 3/09, 

12/10 

Leased cars and trucks 

Motor fuel 

New cars 

7/05-9/05 

7/08-10/08 

12/08-2/09 

6/09 

2/10 

6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09 

1/09-7/09 

8/09 

8/09-11/09 

1/11-6/11 

Effect of interest rates rising to a four year high, 

which impacted the money factor included in 

leases 

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 

Damage to U.S. oil refineries resulting from 

 Hurricane Katrina, along with record global 

 petroleum demand and continued political unrest 

 in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

 Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

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 

“Car Allowance Rebate System” program 

temporarily depressed prices 

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 

None 

None 

7/08, 8/08, 

9/08 
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers 

New cars and trucks 7/08-9/08 

1/09-7/09 

8/09 

8/09-11/09 

1/11-6/11 

Record discounting on vehicles in response to 

 weaker sales and declining economic conditions 

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 

“Car Allowance Rebate System” program 

temporarily depressed prices 

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 

None 

New trucks 

New vehicles 

7/08-9/08 

1/09-7/09 

9/09-11/09 

1/11-6/11 

7/08-9/08 

1/09-7/09 

8/09 

8/09-11/09 

1/11-6/11 

Record discounting on vehicles in response to 

 weaker sales and declining economic conditions 

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 

Record discounting on vehicles in response to 

 weaker sales and declining economic conditions 

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 

“Car Allowance Rebate System” program 

temporarily depressed prices 

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 

None 

None 
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers 

Nonalcoholic beverages 

and beverage materials 

2/05-4/05 

8/08-10/08 

Orange juice futures increased due to cuts in 

projected citrus production of Florida, related to 

the previous hurricane season, combined with a 

small Brazilian coffee crop 

Higher commodity costs combined with a declining 

 demand for soft drinks 

3/07, 4/08, 

3/09, 12/10 

Nondurables 6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09 

Damage to U.S. oil refineries resulting from 

Hurricane Katrina, along with record global 

petroleum demand and continued political unrest 

in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

None 

Nondurables less food 6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09 

Damage to U.S. oil refineries resulting from 

Hurricane Katrina, along with record global 

petroleum demand and continued political unrest 

in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

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 

6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09 

6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09 

Damage to U.S. oil refineries resulting from 

Hurricane Katrina, along with record global 

petroleum demand and continued political unrest 

in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

Damage to U.S. oil refineries resulting from 

Hurricane Katrina, along with record global 

petroleum demand and continued political unrest 

in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing 

None 

None 

 
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers 

Nondurables less food, 

beverages, and apparel 

6/05-9/05 

10/05-11/05 

8/06-10/06 

9/08-12/08 

6/09

Damage to U.S. oil refineries resulting from 

Hurricane Katrina, along with record global 

petroleum demand and continued political unrest 

in Iraq, Venezuela and Nigeria 

Recovery of domestic oil refineries in the Gulf of 

Mexico, combined with easing crude oil prices 

Collapse in profit margins for oil refineries 

Response in crude oil markets to world-wide 

 economic downturn 

Return to normal pricing

None 

Other lodging away from 

home including hotels and 

motels 

Prescription drugs 

Public transportation 

None 

1/07-3/07 

2/08-7/08 

7/08-2/09 

8/09-12/09 

None 

Significantly lower prices for some generic drugs due 

to patent loss and new generic drug discount 

program implementations at several major 

retailers 

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. 

9/05 

None 

None 

Used Cars and Trucks 

Utilities and public 

transportation 

9/08-3/09 

7/09-12/09 

2/11-8/11 

8/05-10/05 

1/06 

2/08-7/08 

7/08-11/08 

Decrease in demand due to the recession and large 

increases in gas prices, and consumers 

experiencing difficulty in getting loans due to the 

global credit crunch 

Decrease in supply due to “Car Allowance Rebate 

System” program, and increase in demand due 

to record low inventory of new cars. Fewer cars 

coming off lease at this time 

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 

Supply disruptions due to Hurricane Katrina 

Larger than normal increase due to recovery from 

 Hurricane Katrina 

Effect of a widening storage deficit, combined with 

 strong summer demand 

Introduction of lower winter gas rates, and recovery 

 of storage inventories 

None 

None 
CPI series – intervention analysis seasonal adjustment-continued

 

 Level shift 

 Series period(s) Event Outliers 

Utility (piped) gas service 8/05-10/05 

3/08-7/08 

8/08-11/08 

3/09-5/09 

Supply disruptions due to Hurricane Katrina 

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 ago, 

weak demand and reduced spot market prices 

10/06