Market Efficiency

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DEFINITION of 'Market Efficiency'

The degree to which stock prices reflect all available, relevant information. Market efficiency was developed in 1970 by Economist Eugene Fama who's theory efficient market hypothesis (EMH), stated that it is not possible for an investor to outperform the market because all available information is already built into all stock prices. Investors who agree with this statement tend to buy index funds that track overall market performance.

BREAKING DOWN 'Market Efficiency'

Investors and academics have a wide range of viewpoints on how efficient the market actually is, as reflected in the strong, semi-strong and weak versions of the EMH. At the other end of the spectrum from Fama and his followers are the value investors, who believe that stocks can become undervalued, or priced below what they are actually worth. Successful value investors make their money by purchasing stocks when they are undervalued and selling them when their price rises to meet or exceed their intrinsic worth.

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