CPI can often overestimate inflation rate due to two different reasons. As prices change throughout the years, consumer behavior also adjust accordingly. Since CPI uses a fixed set of goods and quantities in the market basket, it cannot accurately reflect the change in buying patterns. As a result, the market basket needs to be adjusted occasionally to reasonable track actual consumer needs, though a lag may still exist. In addition, CPI also does not reflect the changes in quality of goods, which, more often than not, change for the better as firms need to improve their products for competition.