Inaccuracies of the Consumer Price Index (CPI) The Consumer Price Index is a measure of the prices of a fixed market basket of some 300 consumer goods and services purchased by a "typical" urban consumer. The 1982-1984 period serves as the base period so analysts can compare other year's changes with this base period. The composition of the market basket is fixed in the base period and is assumed not to change from one period to another. The reason for the assumption is because the CPI measures the costliness of a constant standard of living. ....