The efficient-market hypothesis EMH is a hypothesis in financial economics that states that asset prices reflect all available information.
A direct implication is that it is impossible to "beat the market" consistently on a risk-adjusted basis since market prices should only react to new information. The idea that financial market returns are difficult to predict goes back to Bachelier ,  Mandelbrot ,  and Samuelson Despite its lack of testability, the EMH still provides the basic logic for modern risk-based theories of asset prices. Indeed, modern frameworks such as consumption-based asset pricing and intermediary asset pricing can be thought of as the combination of a model of risk with the hypothesis that markets are efficient.
Suppose that a piece of information about the value of a stock say, about a future merger is widely available to investors. If the price of the stock does not already reflect that information, then investors can trade on it, thereby moving the price until the information is no longer useful for trading.
Note that this thought experiment does not necessarily imply that stock prices are unpredictable. For example, suppose that the piece of information in question says that a financial crisis is likely to come soon. Investors typically do not like to hold stocks during a financial crisis, and thus investors may sell stocks until the price drops enough so that the expected return compensates for this risk.
How efficient markets are and are not linked to the random walk theory can be described through the fundamental theorem of asset pricing. This theorem states that, in the absence of arbitrage, the price of any stock is given by [ clarification needed ]. Note that this equation does not generally imply a random walk.
Efficient Market Hypothesis and Random Walk Theory in Black Scholes
However, if we assume the stochastic discount factor is constant and the time interval is short enough so that no dividend is being paid, we have. Research by Alfred Cowles in the s and s suggested that professional investors were in general unable to outperform the market.
During the ss empirical studies focused on time-series properties, and found that US stock prices and related financial series followed a random walk model in the short-term. In their seminal paper, Fama, Fisher, Jensen, and Roll propose the event study methodology and show that stock prices on average react before a stock split, but have no movement afterwards.
In Fama's influential review paper, he categorized empirical tests of efficiency into "weak-form", "semi-strong-form", and "strong-form" tests. Indeed, Fama later said he "came to regret" using these terms. Despite the disappearance of this terminology from academic journals, undergraduate textbooks continue to emphasize weak vs strong forms of efficiency, as do investing websites like Investopedia. These categories of tests refer to the information set used in the statement "prices reflect all available information.
Semi-strong form tests study information beyond historical prices which is publicly available. Strong-form tests regard private information. Benoit Mandelbrot claimed the efficient markets theory was first proposed by the French mathematician Louis Bachelier in in his PhD thesis "The Theory of Speculation" describing how prices of commodities and stocks varied in markets.
But the work was never forgotten in the mathematical community, as Bachelier published a book in detailing his ideas,  which was cited by mathematicians including Joseph L. Doob , William Feller  and Andrey Kolmogorov.
The efficient markets theory was not popular until the s when the advent of computers made it possible to compare calculations and prices of hundreds of stocks more quickly and effortlessly. In , F. Hayek argued that markets were the most effective way of aggregating the pieces of information dispersed among individuals within a society.
Given the ability to profit from private information, self-interested traders are motivated to acquire and act on their private information. In doing so, traders contribute to more and more efficient market prices. In the competitive limit, market prices reflect all available information and prices can only move in response to news.
Thus there is a very close link between EMH and the random walk hypothesis. The efficient-market hypothesis emerged as a prominent theory in the mids. Paul Samuelson had begun to circulate Bachelier's work among economists.
In Bachelier's dissertation along with the empirical studies mentioned above were published in an anthology edited by Paul Cootner. The paper extended and refined the theory, included the definitions for three forms of financial market efficiency : weak, semi-strong and strong see above. Investors, including the likes of Warren Buffett ,  and researchers have disputed the efficient-market hypothesis both empirically and theoretically.
Behavioral economists attribute the imperfections in financial markets to a combination of cognitive biases such as overconfidence , overreaction, representative bias, information bias , and various other predictable human errors in reasoning and information processing.
These errors in reasoning lead most investors to avoid value stocks and buy growth stocks at expensive prices, which allow those who reason correctly to profit from bargains in neglected value stocks and the overreacted selling of growth stocks. Behavioral psychology approaches to stock market trading are among some of the more promising [ citation needed ] alternatives to EMH and some [ which? But Nobel Laureate co-founder of the programme Daniel Kahneman —announced his skepticism of investors beating the market: "They're just not going to do it.
It's just not going to happen. For example, one prominent finding in Behaviorial Finance is that individuals employ hyperbolic discounting.
Random walk hypothesis
It is demonstrably true that bonds , mortgages , annuities and other similar financial instruments subject to competitive market forces do not.
Any manifestation of hyperbolic discounting in the pricing of these obligations would invite arbitrage thereby quickly eliminating any vestige of individual biases. Similarly, diversification , derivative securities and other hedging strategies assuage if not eliminate potential mispricings from the severe risk-intolerance loss aversion of individuals underscored by behavioral finance.
On the other hand, economists, behaviorial psychologists and mutual fund managers are drawn from the human population and are therefore subject to the biases that behavioralists showcase. By contrast, the price signals in markets are far less subject to individual biases highlighted by the Behavioral Finance programme.
Richard Thaler has started a fund based on his research on cognitive biases.
Efficient market hypothesis and random walk pdf writer
In a report he identified complexity and herd behavior as central to the global financial crisis of Further empirical work has highlighted the impact transaction costs have on the concept of market efficiency, with much evidence suggesting that any anomalies pertaining to market inefficiencies are the result of a cost benefit analysis made by those willing to incur the cost of acquiring the valuable information in order to trade on it.
Additionally the concept of liquidity is a critical component to capturing "inefficiencies" in tests for abnormal returns. Any test of this proposition faces the joint hypothesis problem, where it is impossible to ever test for market efficiency, since to do so requires the use of a measuring stick against which abnormal returns are compared —one cannot know if the market is efficient if one does not know if a model correctly stipulates the required rate of return.
Consequently, a situation arises where either the asset pricing model is incorrect or the market is inefficient, but one has no way of knowing which is the case.
The performance of stock markets is correlated with the amount of sunshine in the city where the main exchange is located. A key work on random walk was done in the late s by Profs. Andrew Lo and Craig MacKinlay; they effectively argue that a random walk does not exist, nor ever has.
While event studies of stock splits are consistent with the EMH Fama, Fisher, Jensen, and Roll, , other empirical analyses have found problems with the efficient-market hypothesis. Early examples include the observation that small neglected stocks and stocks with high book-to-market low price-to-book ratios value stocks tended to achieve abnormally high returns relative to what could be explained by the CAPM.
These risk factor models are not properly founded on economic theory whereas CAPM is founded on Modern Portfolio Theory , but rather, constructed with long-short portfolios in response to the observed empirical EMH anomalies.
What Is the Efficient Market Hypothesis?
For instance, the "small-minus-big" SMB factor in the FF3 factor model is simply a portfolio that holds long positions on small stocks and short positions on large stocks to mimic the risks small stocks face.
These risk factors are said to represent some aspect or dimension of undiversifiable systematic risk which should be compensated with higher expected returns.
See also Robert Haugen. Economists Matthew Bishop and Michael Green claim that full acceptance of the hypothesis goes against the thinking of Adam Smith and John Maynard Keynes , who both believed irrational behavior had a real impact on the markets. Economist John Quiggin has claimed that " Bitcoin is perhaps the finest example of a pure bubble", and that it provides a conclusive refutation of EMH. Tshilidzi Marwala surmised that artificial intelligence influences the applicability of the theory of the efficient market hypothesis in that the more artificial intelligence infused computer traders there are in the markets as traders the more efficient the markets become.
Warren Buffett has also argued against EMH, most notably in his presentation The Superinvestors of Graham-and-Doddsville , saying the preponderance of value investors among the world's best money managers rebuts the claim of EMH proponents that luck is the reason some investors appear more successful than others.
In his book The Reformation in Economics , economist and financial analyst Philip Pilkington has argued that the EMH is actually a tautology masquerading as a theory.
When pressed, proponents will then say [ citation needed ] that any actual investor will converge with the average investor given enough time and so no investor will beat the market average. But Pilkington points out that when proponents of the theory are presented with evidence that a small minority of investor do, in fact, beat the market over the long-run, these proponents then say that these investors were simply 'lucky'.
Pilkington argues that introducing the idea that anyone who diverges from the theory is simply 'lucky' insulates the theory from falsification and so, drawing on the philosopher of science and critic of neolcassical economics Hans Albert , Pilkington argues that the theory falls back into being a tautology or a pseudoscientific construct.
Paul Samuelson argued that the stock market is "micro efficient" but not "macro efficient", in that the EMH is much better suited for individual stocks than it is for the aggregate stock market.
Research based on regression and scatter diagrams has strongly support Samuelson's dictum. The financial crisis of —08 led to renewed scrutiny and criticism of the hypothesis. At the International Organization of Securities Commissions annual conference, held in June , the hypothesis took center stage.
Martin Wolf , the chief economics commentator for the Financial Times , dismissed the hypothesis as being a useless way to examine how markets function in reality. Paul McCulley , managing director of PIMCO , was less extreme in his criticism, saying that the hypothesis had not failed, but was "seriously flawed" in its neglect of human nature. The financial crisis led Richard Posner , a prominent judge, University of Chicago law professor, and innovator in the field of Law and Economics, to back away from the hypothesis.
Posner accused some of his Chicago School colleagues of being "asleep at the switch", saying that "the movement to deregulate the financial industry went too far by exaggerating the resilience—the self healing powers—of laissez-faire capitalism.
Despite this, Fama has conceded that "poorly informed investors could theoretically lead the market astray" and that stock prices could become "somewhat irrational" as a result. The theory of efficient markets has been practically applied in the field of Securities Class Action Litigation. Efficient market theory, in conjunction with " fraud-on-the-market theory ", has been used in Securities Class Action Litigation to both justify and as mechanism for the calculation of damages.
Random Walk Theory
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