The Bureau does not have the time or resources to price every good or service in every retail outlet in every urban area of the country. Therefore, a scientifically selected sample must be used to make the CPI representative of the prices paid for all goods and services purchased by consumers in all urban areas. In fact, the CPI is developed from a series of interrelated samples. These include a Consumer Expenditure Survey from a national sample of almost 29,000 families, which provides detailed information on spending habits. Thus BLS constructs the CPI market basket of goods and services and assigns each item a weight or importance based on total family expenditures. Another national sample of about 24,000 families serves as a point-of-purchase survey, which identifies the places where households purchase various types of goods and services. Finally, BLS uses current Census of Population data to select the urban areas where prices are collected and determine the housing units that are eligible for use in the shelter component of the CPI.
Clearly, extensive research and preparation pave the way for the field representatives' visits to collect price information. Once the information has been collected, it is immediately transmitted to Washington, D.C. for processing. The information is keypunched, and computer-generated listings of data that are outside the bounds of BLS expectations are distributed to BLS commodity specialists who have detailed knowledge about the goods or services priced. Data are checked for accuracy and consistency and corrected or adjusted if necessary. Next, computer programs calculate weighted changes in prices for the different components and areas. After further review by commodity analysts, other computer programs aggregate the data and convert them into indexes that show price changes for each category of items, each local area and the United States. Only then do BLS economists analyze the data and prepare the written reports that are used by other government agencies, the media and the public. The entire process of keying, reviewing, analyzing, and publishing is finished about 20 days after the last data are collected.
While the CPI is still being reported by T.V. and newspapers around the country, BLS field representatives are already hard at work collecting price information for the next month.