1 Third Workshop on ICP Western Asia Beirut, 11-15 October 2004 Design of ICP price survey Sultan Ahmad, Consultant [email protected] Based on Keith.

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Third Workshop on ICP Western Asia

Beirut, 11-15 October 2004

Design of ICP price survey

Sultan Ahmad, [email protected]

Based on Keith Blackburn, Australian Bureau of Statistics

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Aim of this session

Discuss how the actual scope for your country can be determined:

geographic scope

time scope

Clarify choices needed when designing the ICP price survey for your country

Identify possible frame sources

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Scope – defining the targetTarget or ‘ideal’ price

“National annual average price”• Determined by need to link to GDP values

Arithmetic average price of all transactions• For the whole country• For the whole year

Necessary to sample• Products (what products are purchased; product list)• Outlets (where the purchases are made; outlet sample)• Transactions (when the purchases are made; time

sample)

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ErrorsNational annual average price is an estimate

subject to sample errorsubject to many non-sample errors

Product and transaction samples are purposiveTherefore not possible to calculate sample error

Outlet sample may be PPS (probability proportional to size)

If it is, then it is possible to calculate sample errorProduct PPPs result of two estimated averages

May calculate sample error of the ratio

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Probability sample

Random or probability samples preferredUnbiasedEfficient – have minimum variance

Prior information neededAllow stratificationSelection using probability proportional to size (PPS)

PPS size measure (for ICP purposes)Ideally want product sales volumesIn practice may use ‘total sales’ or ‘staff numbers’

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Purposive sample

Subjective selection by person or agencyCPI – used extensively• Could use random sampling for products but don’t

ICP – used for product and transaction samples

Random or probability sampling preferredPurposive sampling used • When information not available for random sampling

– When the cost of random sampling too high

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Product sample

Regional ICP product list provides the ‘frame’

Your list prepared at finalisation workshops provides national sample:

includes ‘representative’ and ‘available’ items

excludes ‘not available’ items – keep to minimum

avoid changes after survey starts (costs/data quality)

May include items not included in CPI

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Outlet sample

Regional circumstances vary enormouslyGeographically

• City states/Single state countries/Multi-state countries

Available CPI infrastructure• single national systems/separate provincial systems• highly developed systems/simple limited-scope

systems

Outlet survey designs should reflect individual national circumstances

Remember CPI design may not be adequate

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Choosing outletsMust choose both location and type

Location – city/rural poor province/wealthy provinceType – street stall, small shop, department store

Outlet choice more important for ICPPrice levels differ by outlet type – cause ICP bias CPI more robust – only needs consistent movements

Care to avoid price collectors taking easy options

Pricing in department stores easierRisk bias if high price goods easier to measure

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Informal marketsSampling frames may exclude informal outlets

e.g. Street stalls and itinerant sellers

Weekly markets may be very important and account for major part of household purchasesInformal markets may require location sample

Price collectors then select specific sellers

Bargaining – no fixed pricesSpecial care needed by price collectors With such markets there is risk of serious bias

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Using CPI frame Is suitable national

CPI frame available?

No

Supplement CPI frame

No

Develop new ICP frame

Use existing CPI frame

Yes

Is a new frame required?

Yes

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Time sample

Aim at average prices for the whole year

Need quantity weighted transaction price

Choices - Monthly/Quarterly/Annual

Frequency – determined by variability

• Type of charge (annual, seasonal etc.)/inflation

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Sample plan

J A S O N D J F M A M J

Food

Cities X X X X

Other urban X X X X

Rural X X X X

Clothing

Cities X X

Other urban X X

Rural X X

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Multiple CPI outlet frames

Multi-state countries may have many independent CPIs (eg India, China)

Design of a single national ICP sample will need input from skilled sampling experts

A purpose-built sample needed

Outlets should be selected on PPS basis

Stratified sample may be appropriate

Use expenditure weights for PPS, if available

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Alternative-frames

If CPI frame not available or needs supplementing alternative frames are needed National level sampling frames

• May provide urban/rural, area and other broad level weights

• National frames may not cover outlets

Local level outlet sampling frames • Usually are available

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Alternative-frame issues

Is it available for statistical purposes?

Is it available in the required time-frame?

Is it reliable?

Is it current?

Does it have the required coverage?

Does it include size indicators?

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Sources of national frames

Retail trade census

Survey of retail sales

Value Added Tax (VAT) records

Business registers

Telephone directories (“Yellow

Pages”)

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Sources of local frames

Local government records

Property tax

Other regulatory systems

Local chambers of commerce

Employer organisations

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 Retail trade census or survey

Frames may exist at national level

as well as local level

Useful for probability proportional

to size (PPS) sampling

Usually maintained by the

statistics office

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Value Added Tax (VAT)

VAT administrative records provide detailed lists of businesses

Up to dateMay not be available for statistical purposesMay support PPS sampling (provide size details)May exclude some sectors such as foodMay exclude informal sector businesses

If available, care needed with exclusionsMay require supplementary sources

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Business registers

Business registers maintained for: Statistical purposes

Administrative purposes

May contain retail outlet locations/addresses

May include size measures sales volume

number of employees

Useful for PPS sampling

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Telephone/business directories

“Yellow Pages” and similar directories

Do not include size measures

May include detailed lists of businesses

May not be comprehensive

• PPS not possible

• simple random sampling can be used

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Thank you

Any questions?