The Efficient Market Theory says that security prices correctly and almost immediately reflect all information and expectations. It says that you cannot consistently outperform the stock market due to the random nature in which information arrives and the fact that prices react and adjust almost immediately to reflect the latest information. Therefore, it assumes that at any given time, the market correctly prices all securities. The result, or so the Theory advocates, is that securities cannot be overpriced or underpriced for a long enough period of time to profit therefrom.

The Theory holds that since prices reflect all available information, and since information arrives in a random fashion, there is little to be gained by any type of analysis, whether fundamental or technical. It assumes that every piece of information has been collected and processed by thousands of investors and this information (both old and new) is correctly reflected in the price. Returns cannot be increased by studying historical data, either fundamental or technical, since past data will have no effect on future prices.

The problem with both of these theories is that many investors base their expectations on past prices (whether using technical indicators, a strong track record, an oversold condition, industry trends, etc). And since investors expectations control prices, it seems obvious that past prices do have a significant influence on future prices.

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