The market basket purchased by the average household in the surveyed population is constantly changing, not only in response to price changes, but also in response to changes in incomes, tastes, and availability of new products and services not previously available to consumers.
Population coverage.
Sampling problem
Sub situation bias: occurs because
Prices of the base year goods and services increase,
The consumer will choose a different set of goods and services that are less expensive.
Quality changes and new products
The BLS (Bureau of Labor Statistics) continues to prices of the base year goods and service and assumes that the consumer will purchase the same quantity of the item even though its price has risen.
In fact, consumers will substitute away from those items for which price is rising more rapidly and this will lessen, to some extent, the impact of the general price increase