The CPI is a measure of the change in the cost of living between two periods in time. For example, if an average family had to spend $50 for groceries in 1988 and $150 for those same groceries in 1998, the CPI would have tripled in 10 years. BLS measures changes in these costs from year to year and reports on them in the form of two Consumer Price Indexes, the CPI-W and the CPI-U.
The CPI-W is the older index. It covers only urban wage earners and clerical workers and represents about 32 percent of the U.S. population. The CPI-U is the newer, broader index. It began in 1978 and represents the buying habits of about 80 percent of the U.S. population. It includes wage earners; clerical workers; professional, managerial, and technical workers; the self-employed; short-term workers; the unemployed; retirees; and others not in the labor force.