An index number measures relative changes in price, quantity, or value over time. It can help frame policies, reveal trends, and deflate values. Index numbers are classified by what they measure (price, quantity, value) and their construction method (simple, weighted aggregate, weighted average of price relatives). Common uses include analyzing purchasing power, incomes, and markets. Weighted indexes assign weights to items, with methods including Laspeyres, Paasche, Fisher, and Kelly. Consumer price indexes measure price changes in consumer baskets.