There are three forms of efficient-market hypothesis, namely, the weak form, the semistrong form and the strong form.
The weak form says that the current prices of stocks already fully reflect all the information that is contained in the historical sequence of prices. The weak form of the efficient-market hypothesis is popularly known as the random-walk theory.
The semistrong form of the efficient-market hypothesis says that current prices of stocks not only reflect all informational content of historical prices but also reflect all publicly available knowledge about the corporations being studied. The semistrong form of the efficient-market hypothesis maintains that as soon as information becomes publicly available, it is absorbed and reflected in stock prices.
The strong form of the efficient-market hypothesis maintains that not only is publicly available information useless to the investor or analyst but all information is useless. The strong form of the efficient-market hypothesis maintains that no information that is available, be it public or inside can be used to earn consistently superior investment returns.
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