Final answer:
HHI over 1800 indicates high concentration and lack of competition in a market. It is used in antitrust analysis and can lead to a merger being challenged by the FTC.
Step-by-step explanation:
HHI over 1800 indicates high concentration in a market and suggests a lack of competition. It is used as a measure of market concentration and is commonly used in antitrust analysis. If a merger would result in an HHI of more than 1800, it is likely to be challenged by the Federal Trade Commission (FTC).