Any price index compares the amount something would cost in a given year to the amount it would cost in the base year, a benchmark year that serves as a basis for comparison for prices in other years. In the case of the GDP deflator, the something being compared is the value of all goods and services produced in the economy this year:
GDP deflator | = | (Value of all goods and services produced in the economy this year using this year's prices)×100Value of all goods and services produced in the economy this year using the base year's prices |
However, the CPI measures the overall cost of goods and services purchased by a typical consumer. There are many types of domestically produced goods, such as tanks and commercial jets, that are not purchased by the typical consumer. There are also many foreign goods and services, such as foreign cars and clothing, that are purchased by domestic consumers. To accurately measure consumer prices, the government must filter out many types of domestically made products and include many types of foreign-made products.