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At What Price?: Conceptualizing and Measuring Cost-of-Living and Price Indexes
for the C&S Survey are selected with a systematic sampling procedure, with probability of selection proportional to the amount of expenditure in that PSU group and its item stratum. This systematic sampling procedure guarantees that over the 4-year period each of the ELIs will be selected for pricing. The outlets actually sampled from each frame are selected independently for each PSU group and POPS category, with probability of selection proportional to the amount of expenditure in that PSU group and POPS category. To give readers a sense of the number of outlets selected, the largest number is nine, in the POPS foods and beverages category, PSU group Philadelphia.
At a selected outlet a BLS field representative uses a multistage probability selection procedure for selecting the specific item to be priced among those that the outlet sells that fall within the designated-for-pricing ELI definition. The probability of selection is, if the information is available, proportional to the sales of the items in the ELI groups. Otherwise it is either based on the proportion of shelf space or, as a last resort, assigning equal probability to each item. Once the item is selected, its price is recorded. These are the prices that are weighted and used in the computation of the Rhzt used in the CPI computation.