Efficient Market Hypothesis (EMH)

This is the age of information, where information is more powerful than any weapon. The markets are changing rapidly and information is the key to identifying such changes and reacting to them in the best possible way. Information gives us the business a path to follow, targets to achieve and a competitive advantage, if used correctly.

Efficient Market Hypothesis (EMH) defines three levels of information availability in a market (mainly financial markets).

1. Strong Market
2. Semi-Strong Market
3. Weak Market

Strong Market

EMH says that in a strong market perfect information exists and is highly efficient in information distribution. This means that, in a market, all past information, present information, all disclosed information and all undisclosed information are publicly available. In practice, this is highly unlikely and could be concluded that such markets do not exist. If such markets exist, everybody would be making unlimited and continuous profits within markets, and that is not practical.

The prices of financial instruments trading in these markets will instantly reflect the availability of undisclosed information (insider information). 

Semi-Strong Market

EMH defines a semi-strong market as one with all past information, present information and all disclosed information. Typically this is the type of market that exists most of the time. Businesses will do anything to prevent important information leaking out to the market  since that gives their competitors an enormous advantage. So undisclosed information stays undisclosed and publicly hidden.

The prices of financial instruments trading in these markets will instantly reflect the availability of public information. 

Weak Market

EMH says that Weak Markets only give out the past disclosed information to the public. Decision making can be tough and rigid in these types of markets.

In these markets prices of financial instruments will only reflect based on the past information.

Sources : http://en.wikipedia.org/wiki/Efficient-market_hypothesis