Workshop on Price Index Compilation Issues

February 23-27, 2015

Sample Design, Selection, and Maintenance

Gefinor Rotana Hotel, Beirut, Lebanon

Sample Design and Selection

Lecture outlineIntroductionCommon problems in price survey samplingWhere to startSample designSample selection and recruitment

Sample Design and Selection

IntroductionUse of statistically sound sampling techniques vs. judgmental samplesApproach dependent on availability of dataNeed to maintain sample size

Sample Design and Selection

Common Problems in Price SamplingWithout probability sampling methods it is difficult to measure accuracy and optimize sampleAttrition of sampled establishments and products at the same time as new goods appearLack of up-to-date sampling frameVoluntary nature of most price surveysPurposive sampling increases chance of bias

Sample Design and Selection

Where to StartObjectives of the surveys measure of inflation, use as deflator monthly or quarterly estimates level of detail for publication important commodities, groupings, industries

Sample Design and Selection

Coverage National, regional, urban only, selected areas Economic activities (industry, services, etc.) Income / population groups (e.g., low, middle, high income households; family size; military)

Sample Design and Selection

Data to be collected Purchaser, producer, basic price?Type of price (transaction, list, average, cost plus)Time of recording (cash or accrual)Detailed specification of the product which includes all the terms involved in the purchaseTemporary non-availability of some pricesPoint in time price or average over the period

Sample Design and Selection

Sampling frames Business register (complete and up-to-date?) Household lists Stratification variables included (ISIC, output or sales, location; income, household expenditures, etc.)

Sample Design and Selection

Level of accuracy and sample size Probability sampling enables us to use population variance to determine sample size Non-probability sampling must be applied carefully to ensure adequate representation

Sample Design and Selection

Available resources such as staff involved and method of collection are constraintsLegislative issues such as voluntary or mandatory reporting and laws governing confidentiality or privacy affect our procedures

Sample Design and Selection

Sample Design Probability vs. non-probability

Simple random sampling Stratified random sampling Probability proportionate to size (PPS) Cutoff samples

Sample Design and Selection

 Sample frame selection business register, business census,

industry surveys population census, housing lists,

household surveys

Sample structure and stratification  PPI, XMPI—region, industry, establishment, product

CPI—area, item, outlet, variety

 

Sample Design and Selection

Sample allocation—what will resources allow? XMPI, PPI by product and industry; CPI by area and item specific products, industries, items that should be published cutoff sampling may be best approach

Sample Design and Selection

Sample Selection and Recruitment Specific issues in the CPI sample design selection of geographic areas (cities and rural areas) selection of outlets and/or markets for pricing selection of items to include in the basket number of price observations per item frequency of price collection per item (often determined by price variability) selection of individual varieties to represent items

Sample Design and Selection

Specific issues in the XMPI, PPI sample design selection of industrial coverage (mining, manufacturing, energy, agriculture, transport, etc.) selection of representative industries selection of representative establishments selection of representative products/commodities frequency of price collection per products selection of individual transactions to represent products

Sample Design and Selection

Recruitment of establishments/outlets via personal visit Selecting transactions / varieties for pricing • identify important price determining

characteristics• record the detailed specifications on

collection document

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