Friday, October 17, 2008

Markets Are Efficient But Can Be Wrong

Markets are efficient and the press and the other media often ignore or misstate the concept's meaning and ideas.

A market is efficient if past prices cannot be used to predict future prices. The period to period change in the prices of actively traded assets such as stocks is random and it is this randomness that makes stock efficient.

In an efficient market, as in all other random markets, the best basis for predicting tomorrow's price is today's price plus a gain for the risk of buying the stock. Likewise, the price of the asset traded in an efficient market already reflects the effects of any information that is available whether through announcement, deduction, logic or expectation. Numerous studies over the past fifty years have upheld the efficiency of actively traded markets.

So any statement about the future movement of a stock's price based on its previous stock price movement is hogwash no matter the terminology used such as top, bottom, stochastic, candlestick, capitulation, and many others. Likewise, any statement that a stock price does not reflect announced information or that it will take several days for the stock price to reflect the news is also hogwash.

The concept of efficient markets does not mean that a market's prediction about a stock's price is always correct. The price in an efficient market is the market's best guess based on all knowable information. The market can guess wrong. Just as in any prediction about future events that have not taken place yet, a guess can be wrong or unexpected events can happen. In an efficient market, the price of a stock is fair in that it is just as likely to be too high as it is too low and no historical information about past stock prices or past events can give an investor an edge over a naïve investor who just buys and holds.

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