An index, specifically a price index (e.g., Consumer Price Index or Producer Price Index), is a normalized statistical measure designed to represent the average change in the price of a group of goods or services over time. It provides a standardized way to compare price levels between different periods (e.g., months or years) by setting a base period to 100 and showing subsequent changes relative to that base. This makes it the precise tool for expressing price changes over time.

B. Inflation: Inflation is the economic phenomenon or rate of increase in the general price level. An index is the tool used to measure and express inflation, not the expression itself.

C. Variance: Variance is a statistical measure of data dispersion or the difference between planned and actual costs. It does not specifically track price trends over time.

D. Discrepancy: This is a general term for a difference or inconsistency and is not a specific tool used in economics or supply management for tracking price changes.

1. International Monetary Fund (IMF). (2020). Producer Price Index Manual: Concepts and Practices. "Chapter 1: An Introduction to Price Index Numbers." The manual states

"Index numbers are used to provide summary measures of the price...movements of a heterogeneous collection of commodities." This defines an index as the tool for measuring price changes. [URL: https://www.imf.org/en/Data/Statistics/ppi-manual]

2. Mankiw

N. G. (2021). Principles of Economics (9th ed.). Cengage Learning. Chapter 24

"Measuring the Cost of Living

" explains that the Consumer Price Index (CPI) is the measure economists use to monitor changes in the cost of living over time

i.e.

to express price changes.

3. MIT OpenCourseWare. (2018). 14.01 Principles of Microeconomics

Lecture 23: Inflation. The course defines the Consumer Price Index (CPI) as a measure of the overall cost of goods and services

which is used to monitor how prices change over time. [URL: https://ocw.mit.edu/courses/14-01-principles-of-microeconomics-fall-2018/resources/mit1401f18lec23/]