What primarily distinguishes strong form EMH from weak and semi-strong forms?

The strong form of the Efficient Market Hypothesis (EMH) is distinguished from the weak and semi-strong forms primarily by its inclusion of insider information. This means that according to the strong form, all information—public and private (non-public insider information)—is fully reflected in stock prices. Therefore, no investor can achieve excess returns by using any kind of information, including insider information.

In contrast, weak form EMH states that all past trading information is reflected in stock prices, thus only past price movements can be used to forecast future prices. Semi-strong form EMH states that all publicly available information is reflected in stock prices, meaning that while investors cannot gain an edge using public data, they may still possess an advantage if they have access to private information. The strong form therefore is the most robust in terms of market efficiency as it asserts that even those with insider knowledge cannot outperform the market. This fundamental aspect is what distinctly characterizes the strong form within the hierarchy of the EMH.