Because the consumer price index reflects the goods and services bought by consumers better than the GDP deflator does,it is the more common gauge of inflation.
By keeping the basket of goods and services the same when computing the CPI,the Bureau of Labor Statistics isolates the effects of price changes from the effect of any quantity changes that might be occurring at the same time.
The CPI for 2008 is computed by dividing the price of the basket of goods and services in 2008 by the price of the basket of goods and services in the base year,then multiplying by 100.