Efficient_Market_Hypothesis

Market Efficiency and Anomalies Professor Lasse H. Pedersen Prof. Lasse H. Pedersen 1 Outline Versions of the Efficient Market Hypothesis (EMH) Random Walk What makes the market efficient' Problems with testing EMH Evidence in favor of EMH Evidence against EMH: Anomalies Prof. Lasse H. Pedersen 2 Versions of the Efficient Market Hypothesis (EMH) Weak-form efficiency: – prices reflect all information contained in past trading Semistrong-form efficiency: – prices reflect all publicly available information Strong-form efficiency: – prices reflect all relevant information, including inside information According to each of these theories, which kind of information cannot be used to trade profitably' Prof. Lasse H. Pedersen 3 Random Walk EMH implies that: a stock price is always at the “fair” level (fundamental value) a stock price reacts to news immediately a stock price changes only when the fair level changes therefore, stock price changes are unpredictable because no one knows tomorrow’s news that is, the stock price is a “random walk”: – tomorrow, the price can go either up or down if the price must go up tomorrow – what would happen today' – the risk-adjusted likelihood of up- and down-movements are equal if the price were extremely likely to go up tomorrow – what would happen today' Prof. Lasse H. Pedersen 4 What Makes the Market Efficient' The market is made efficient by supply and demand pressures – If it is not efficient, investors will trade to take advantage of the inefficiencies But, if the market is already efficient – no one will expend resources on security analysis – every investor should just buy a mix of the risk-free security and the market portfolio Grossman-Stiglitz paradox: How can market be efficient if no one makes security analysis' – Two economists walk down the street and spot a $20 bill. One starts to pick it up, but the other one says: “don’t bother; if the bill were real, someone would have picked it up already.” Prof. Lasse H. Pedersen 5 Economies of Scale in Securities Research If “beating the market” is possible, it is difficult To do so requires resources (time, money, data, computer power, insight, ideas, etc.) Economies of scale: expend resources only if – you have a lot of money, or – you manage a lot of money Small investors can beat the market only if there are “doubly inefficient markets”: – the financial market is inefficient – the market for money management is inefficient Prof. Lasse H. Pedersen 6 Problems with Testing EMH The lucky-event issue – if many people play the lottery someone will win. This does not imply that the winner has “ability.” – if many people try to predict the future, someone will be right. Data mining – many researchers and market participants are looking for patterns in the data – even truly random samples, however, appear to have patterns (“hot-hand hypothesis”) – in-sample predictability need not imply out-of-sample predictability. The joint-test issue – is the market inefficient or did you adjust for risk incorrectly' Prof. Lasse H. Pedersen 7 Evidence in Favor of EMH Stock prices are close to random walks Stock returns have low serial correlation Stock returns are very hard to predict Portfolio managers – do not beat the market on average – almost no one beats the market consistently Prof. Lasse H. Pedersen 8 Evidence Against EMH: Anomalies Bubbles Momentum Reversals Post-earnings announcement drift Small-firm effect Book-to-market effect Prof. Lasse H. Pedersen 9