The return on a stock beyond what would be predicted by market movements alone is referred to as the

Though the efficient market hypothesis (EMH), as a whole, theorizes that the market is generally efficient, the theory is offered in three different versions: weak; semi-strong; and strong.

The basic efficient market hypothesis posits that the market cannot be beaten because it incorporates all important determining information into current share prices. Therefore, stocks trade at the fairest value, meaning that they can't be purchased undervalued or sold overvalued.

The theory determines that the only opportunity investors have to gain higher returns on their investments is through purely speculative investments that pose a substantial risk.

  • The efficient market hypothesis posits that the market cannot be beaten because it incorporates all important information into current share prices, so stocks trade at the fairest value. 
  • Though the efficient market hypothesis theorizes the market is generally efficient, the theory is offered in three different versions: weak, semi-strong, and strong.
  • The weak form suggests today’s stock prices reflect all the data of past prices and that no form of technical analysis can aid investors. 
  • The semi-strong form submits that because public information is part of a stock's current price, investors cannot utilize either technical or fundamental analysis, though information not available to the public can help investors.
  • The strong form version states that all information, public and not public, is completely accounted for in current stock prices, and no type of information can give an investor an advantage on the market.

The three versions of the efficient market hypothesis are varying degrees of the same basic theory. The weak form suggests that today’s stock prices reflect all the data of past prices and that no form of technical analysis can be effectively utilized to aid investors in making trading decisions.

Advocates for the weak form efficiency theory believe that if the fundamental analysis is used, undervalued and overvalued stocks can be determined, and investors can research companies' financial statements to increase their chances of making higher-than-market-average profits.

The semi-strong form efficiency theory follows the belief that because all information that is public is used in the calculation of a stock's current price, investors cannot utilize either technical or fundamental analysis to gain higher returns in the market.

Those who subscribe to this version of the theory believe that only information that is not readily available to the public can help investors boost their returns to a performance level above that of the general market.

The strong form version of the efficient market hypothesis states that all information—both the information available to the public and any information not publicly known—is completely accounted for in current stock prices, and there is no type of information that can give an investor an advantage on the market.

Advocates for this degree of the theory suggest that investors cannot make returns on investments that exceed normal market returns, regardless of information retrieved or research conducted.

There are anomalies that the efficient market theory cannot explain and that may even flatly contradict the theory. For example, the price/earnings (P/E) ratio shows that firms trading at lower P/E multiples are often responsible for generating higher returns.

The neglected firm effect suggests that companies that are not covered extensively by market analysts are sometimes priced incorrectly in relation to their true value and offer investors the opportunity to pick stocks with hidden potential. The January effect shows historical evidence that stock prices—especially smaller cap stocks—tend to experience an upsurge in January.

Though the efficient market hypothesis is an important pillar of modern financial theories and has a large backing, primarily in the academic community, it also has a large number of critics. The theory remains controversial, and investors continue attempting to outperform market averages with their stock selections.

Due to the empirical presence of market anomalies and information asymmetries, many practitioners do not believe that the efficient markets hypothesis holds in reality, except, perhaps, in the weak form.

The efficient market hypothesis (EMH) is important because it implies that free markets are able to optimally allocate and distribute goods, services, capital, or labor (depending on what the market is for), without the need for central planning, oversight, or government authority. The EMH suggests that prices reflect all available information and represent an equilibrium between supply (sellers/producers) and demand (buyers/consumers). One important implication is that it is impossible to "beat the market" since there are no abnormal profit opportunities in an efficient market.

The EMH has three forms. The strong form assumes that all past and current information in a market, whether public or private, is accounted for in prices. The semi-strong form assumes that only publicly-available information is incorporated into prices, but privately-held information may not be. The weak form concedes that markets tend to be efficient but anomalies can and do occur, which can be exploited (which tends to remove the anomaly, restoring efficiency via arbitrage). In reality, only the weak form is thought to exist in most markets, if any.

To test the semi-strong version of the EMH, one can see if a stock's price gaps up or down when previously private news is released. For instance, a proposed merger or dismal earnings announcement would be known by insiders but not the public. Therefore, this information is not correctly priced into the shares until it is made available. At that point, the stock may jump or slump, depending on the nature of the news, as investors and traders incorporate this new information.

The efficient market hypothesis (EMH), alternatively known as the efficient market theory, is a hypothesis that states that share prices reflect all information and consistent alpha generation is impossible.

According to the EMH, stocks always trade at their fair value on exchanges, making it impossible for investors to purchase undervalued stocks or sell stocks for inflated prices. Therefore, it should be impossible to outperform the overall market through expert stock selection or market timing, and the only way an investor can obtain higher returns is by purchasing riskier investments.

  • The efficient market hypothesis (EMH) or theory states that share prices reflect all information.
  • The EMH hypothesizes that stocks trade at their fair market value on exchanges.
  • Proponents of EMH posit that investors benefit from investing in a low-cost, passive portfolio.
  • Opponents of EMH believe that it is possible to beat the market and that stocks can deviate from their fair market values.

Although it is a cornerstone of modern financial theory, the EMH is highly controversial and often disputed. Believers argue it is pointless to search for undervalued stocks or to try to predict trends in the market through either fundamental or technical analysis.

Theoretically, neither technical nor fundamental analysis can produce risk-adjusted excess returns (alpha) consistently, and only inside information can result in outsized risk-adjusted returns.

The June 27, 2022 share price of the most expensive stock in the world: Berkshire Hathaway Inc. Class A (BRK.A).

While academics point to a large body of evidence in support of EMH, an equal amount of dissension also exists. For example, investors such as Warren Buffett have consistently beaten the market over long periods, which by definition is impossible according to the EMH.

Detractors of the EMH also point to events such as the 1987 stock market crash, when the Dow Jones Industrial Average (DJIA) fell by over 20% in a single day, and asset bubbles as evidence that stock prices can seriously deviate from their fair values.

The assumption that markets are efficient is a cornerstone of modern financial economics—one that has come under question in practice.

Proponents of the Efficient Market Hypothesis conclude that, because of the randomness of the market, investors could do better by investing in a low-cost, passive portfolio.

Data compiled by Morningstar Inc., in its June 2019 Active/Passive Barometer study, supports the EMH. Morningstar compared active managers’ returns in all categories against a composite made of related index funds and exchange-traded funds (ETFs). The study found that over a 10 year period beginning June 2009, only 23% of active managers were able to outperform their passive peers. Better success rates were found in foreign equity funds and bond funds. Lower success rates were found in US large-cap funds. In general, investors have fared better by investing in low-cost index funds or ETFs.

While a percentage of active managers do outperform passive funds at some point, the challenge for investors is being able to identify which ones will do so over the long term. Less than 25 percent of the top-performing active managers can consistently outperform their passive manager counterparts over time.

Market efficiency refers to how well prices reflect all available information. The efficient markets hypothesis (EMH) argues that markets are efficient, leaving no room to make excess profits by investing since everything is already fairly and accurately priced. This implies that there is little hope of beating the market, although you can match market returns through passive index investing.

The validity of the EMH has been questioned on both theoretical and empirical grounds. There are investors who have beaten the market, such as Warren Buffett, whose investment strategy focused on undervalued stocks made billions and set an example for numerous followers. There are portfolio managers who have better track records than others, and there are investment houses with more renowned research analysis than others. EMH proponents, however, argue that those who outperform the market do so not out of skill but out of luck, due to the laws of probability: at any given time in a market with a large number of actors, some will outperform the mean, while others will underperform.

There are certainly some markets that are less efficient than others. An inefficient market is one in which an asset's prices do not accurately reflect its true value, which may occur for several reasons. Market inefficiencies may exist due to information asymmetries, a lack of buyers and sellers (i.e. low liquidity), high transaction costs or delays, market psychology, and human emotion, among other reasons. Inefficiencies often lead to deadweight losses. In reality, most markets do display some level of inefficiencies, and in the extreme case an inefficient market can be an example of a market failure.

Accepting the EMH in its purest (strong) form may be difficult as it states that all information in a market, whether public or private, is accounted for in a stock's price. However, modifications of EMH exist to reflect the degree to which it can be applied to markets:

  • Semi-strong efficiency - This form of EMH implies all public (but not non-public) information is calculated into a stock's current share price. Neither fundamental nor technical analysis can be used to achieve superior gains.
  • Weak efficiency - This type of EMH claims that all past prices of a stock are reflected in today's stock price. Therefore, technical analysis cannot be used to predict and beat the market.

The more participants are engaged in a market, the more efficient it will become as more people compete and bring more and different types of information to bear on the price. As markets become more active and liquid, arbitrageurs will also emerge, profiting by correcting small inefficiencies whenever they might arise and quickly restoring efficiency.