This document discusses index numbers, which are ratios or averages expressed as percentages that assign a single number to represent trends across multiple statistics over time. Index numbers allow comparisons between two or more time periods by using one as a base period for standard comparison. They are specialized averages that measure relative changes from one time or place to another. Constructing index numbers involves selecting data and items, choosing base periods and weights, and selecting an appropriate formula. Index numbers are useful for policymaking, market analysis, and measuring quantitative changes, but have limitations such as sampling errors and unreliable long-term comparisons. The document also covers types of index numbers and various methods for their construction.