Efficient Market Hypothesis: Myth of Reality?

While this definition reflects the main idea of the EMH it might be extended to explain the underlying assumption. For example Malkiel (1992) proposed the following definition:

A capital market is said to be efficient to if it fully and correctly reflects all relevant information in determining security prices. Therefore, more formally, the market is efficient with respect to some information set. ..if security prices would be unaffected by revealing that information to all participants. Moreover, efficiency implies that it is impossible to make economic profits by trading on the basis of the defined information set. Find out more here; hypothesis

As it follows from the Malkiel (1992) definition if the market is efficient the company market value should be an unbiased estimate of the true value. Nevertheless it is important to stress that:

1. Market efficiency does not require that market price is equal to the true value
2. There is an equal probability that stocks over or under valued at any point in the time 
3. And finally, investors should not be able to consistently identify under or over valued stocks using any investment strategy ( Damodaran, 2006).

What are the implications of the market efficiency from the individual investor perspective?

Firstly, equity research is costly and provides no benefits. Secondly strategies that have minimal execution costs such as randomly diversified portfolio or indexing to the market would be superior to any other investment strategy. Thirdly, a strategy that has minimum transaction costs should provide higher returns in the long run (Damodaran, 2006).

Nevertheless it is important to stress that markets are not efficient due to their nature, but they are driven to efficiency by the actions of the investors. Therefore Roberts(1967) distinguished among three forms of the market efficiency:

1. Weak form: the information set includes only historic data.
2. Semi strong: the information set includes publicly available information.
3. Strong form: the information set includes all information know to any market participant and includes private information.

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