The Three Basic Forms of the EMH
The efficient market hypothesis assumes that markets are efficient. However, the efficient market hypothesis (EMH) can be categorized into three basic levels:
1. Weak-Form EMH
The weak-form EMH implies that the market is efficient, reflecting all market information. This hypothesis assumes that the rates of return on the market should be independent; past rates of return have no effect on future rates. Given this assumption, rules such as the ones traders use to buy or sell a stock, are invalid.
2. Semi-Strong EMH
The semi-strong form EMH implies that the market is efficient, reflecting all publicly available information. This hypothesis assumes that stocks adjust quickly to absorb new information. The semi-strong form EMH also incorporates the weak-form hypothesis. Given the assumption that stock prices reflect all new available information and investors purchase stocks after this information is released, an investor cannot benefit over and above the market by trading on new information.
3. Strong-Form EMH
The strong-form EMH implies that the market is efficient: it reflects all information both public and private, building and incorporating the weak-form EMH and the semi-strong form EMH. Given the assumption that stock prices reflect all information (public as well as private) no investor would be able to profit above the average investor even if he was given new information.
Weak, Semi-Strong and Strong EMH