Modern Portfolio Theory - Overall investment strategy that seeks to construct an optimal portfolio by considering the relationship between risk and return, especially as measured by alpha, beta, and R-squared. This theory recommends that the risk of a particular stock should not be looked at on a standalone basis, but rather in relation to how that particular stocks price varies in relation to the variation in price of the market portfolio. The theory goes on to state that given an investors preferred level of risk, a particular portfolio can be constructed that maximizes expected return for that level of risk. also called modern investment theory.

Efficient Market Theory - The now largely discredited theory that all market participants receive and act on all of the relevant information as soon as it becomes available. If this were strictly true, no investment strategy would be better than a coin toss. Proponents of the efficient market theory believe that there is perfect information in the stock market. This means that whatever information is available about a stock to one investor is available to all investors except, of course, insider information, but insider trading is illegal. Since everyone has the same information about a stock, the price of a stock should reflect the knowledge and expectations of all investors. The bottom line is that an investor should not be able to beat the market since there is no way for him/her to know something about a stock that isn't already reflected in the stock's price. Proponents of this theory do not try to pick stocks that are going to be winners; instead, they simply try to match the market's performance. However, there is ample evidence to dispute the basic claims of this theory, and most investors don't believe it.

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Source: investorwords.com