1 third workshop on icp western asia beirut, 11-15 october 2004 design of icp price survey sultan...
TRANSCRIPT
1
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
2
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
3
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)
4
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
5
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’
6
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
7
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
8
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
9
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
10
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
11
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
12
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
13
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
14
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
15
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
16
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?
17
Sources of national frames
Retail trade census
Survey of retail sales
Value Added Tax (VAT) records
Business registers
Telephone directories (“Yellow
Pages”)
18
Sources of local frames
Local government records
Property tax
Other regulatory systems
Local chambers of commerce
Employer organisations
19
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
20
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
21
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
22
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
23
Thank you
Any questions?