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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. estimates the Rthz. An oversimplified version of the BLS procedure is the following: Let a sample of N items be drawn from the universe of items in item stratum z (with replacement), with the probability of selection of item i equal to whi. Then 
is an unbiased estimate of the weighted average version of Rthz, and 
is a consistent estimator of the geometric mean version of Rthz. The BLS then updates its index 
These indexes are aggregated to form indexes for aggregate areas (e.g., U.S. cities), aggregate items (e.g., expenditure classes), or both. Let H denote the aggregate area and Z the aggregate item for which an index is to be formed. The index for this aggregate area and item is calculated as 
where 
and 
where jez denotes the items drawn from the universe of items in item stratum j. Consumer Expenditure SurveyThe CEX, sponsored by BLS and conducted by the Bureau of the Census, is a national probability sample of household units. It is comprised of two parts, a Quarterly Interview Panel Survey and a Diary Survey. Each “consumer unit” in the household selected for the Quarterly Interview Panel Survey is interviewed for 5 consecutive quarters about relatively large expenditure items (e.g., major appliances) and expenditures that occur at regular intervals (e.g., utility bills). A sample of 8,910 addresses are contacted for the Quarterly Interview Panel Survey in each of the calendar quarters, and the number of completed interviews per quarter is targeted at 6,160. Each consumer unit selected for the Diary Survey completes a diary on expenditure information on frequently purchased items and
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for this basic stratum by the chaining formula described earlier, namely,