an important difference between cpi and the gdp deflator is
Like the consumer price index (CPI), the GDP deflator is a measure of price inflation / deflation with respect to a specific base year; the GDP deflator of the base year itself is equal to 100. Unlike the CPI, the GDP deflator is not based on a fixed basket of goods and services; the "basket" for the GDP deflator is allowed to change from year to year with people's consumption and investment patterns.