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Is technical analysis based on some underlying factors in the market or do they work simply because other people use them?
Technical analysis is based more on psychology than anything else. As an example, if an analyst estimates or believes that a stock is undervalued, or simply wants to re-balance their portfolio, then they will buy some amount, moving the price up. Others in the market see the upwards move as the start of an upwards trend, an indication that the stock is undervalued or perhaps even that an insider is trading ahead of better than expected data from the firm. They then buy the stock creating a self-fulfilling prophecy and pulling more traders in as they see an upward trend being confirmed. This is even more pronounced in a bear market as fear is an even stronger driver. When a trader sees a stock is falling they are more likely to jump to the conclusion that it is due to expected poor performance of the firm and that the firm and the economy are both in trouble and going down than to think that it is simply a retrenching or a large investor re-balancing etc. To quote Credit Suisse [1]  A chart is a mirror   of the mood of the crowd and not of the fundamental factors. Thus, technical analysis is the   analysis of human mass psychology. Therefore, it is also called behavioral finance. The underlying truth that makes technical analysis work is that people are predictably irrational, at least in the short run and tend to follow the same patterns of thought. references: [1] https://www.credit-suisse.com/pwp/pb/pb_research/technical_tutorial_de.pdf [2] http://www.amazon.com/The-Psychology-Technical-Analysis-Profiting/dp/1557385432 [3] CFA level 1 syllabus