Index numbers are statistical measures that represent changes in a variable or a group of related variables over time. They are commonly used to track economic indicators, such as the Consumer Price Index (CPI), which measures inflation by comparing the price of a basket of goods and services at different times. These numbers are typically expressed as a percentage, with a base year set to 100. For example, if an index number rises to 120, it indicates a 20% increase compared to the base year. Index numbers help economists, businesses, and policymakers analyze trends and make informed decisions.