EMH

有效市场假说（Efficient Market Hypothesis）又叫效率市场假说，有效市场假设。该假说认为，投资者在买卖股票时会迅速有效地利用可能的信息。所有已知的影响一种股票价格的因素都已经反映在股票的价格中，因此根据这一理论，股票的技术分析是无效的。
An investment theory that states that it is impossible to "beat the market" because stock market efficiency causes existing share prices to always incorporate and reflect all relevant information. According to the EMH, this means that stocks always trade at their fair value on stock exchanges, making it impossible for investors to either purchase undervalued stocks or sell stocks for inflated prices. As such, it should be impossible to outperform the overall market through expert stock selection or market timing, and that the only way an investor can possibly obtain higher returns is by purchasing riskier investments.
由效率市场理论延伸发展，Fama 依市场效率性质提出弱势效率、半强势效率及强势效率，其分述如下：

弱势效率（Weak Form Efficiency）
目前股票价格已充分反应过去股票价格所提供各项情报。所以，投资人无法在运用各种方法对过去股票价格进行分析，在利用分析结果来预测未来股票价格，意即投资者无法再利用过去资讯来获得高额报酬。所以，弱势效率越高，若以过去价量为基础的技术分析来进行预测效果将会十分不准确。
One of the different degrees of efficient market hypothesis (EMH) that claims all past prices of a stock are reflected in today's stock price. Therefore, technical analysis cannot be used to predict and beat a market.

半强势效率（Semi-Strong Form Efficiency）
目前股票价格已充分反应于所有公开资讯上，所以，投资者无法利用情报分析结果来进行股票价格预测而获取高额报酬。因此，半强势效率越高，依赖公开的财务报表、经济情况及政治情势来进行基本面分析，然后再预测股票价格是徒劳无功。
A class of EMH (Efficient Market Hypothesis) that implies all public information is calculated into a stock's current share price. Meaning that neither fundamental nor technical analysis can be used to achieve superior gains.

强势效率（Strong Form Efficiency）
目前股票价格充分反应了所有已公开和未公开之所有情报。虽然情报未公开，但投资者能利用各种管道来获得资讯，所以，所谓未公开的消息，实际上是已公开的资讯且已反应于股票价格上。此种情形下，投资者也无法因拥有某些股票内幕消息而获取高额报酬。
The strongest version of market efficiency. It states all information in a market, whether public or private, is accounted for in a stock price. Not even insider information could give an investor the advantage.