Semi-Strong Form Efficiency

Semi-strong form efficiency is an aspect of the Efficient Market Hypothesis (EMH) that assumes that current stock prices adjust rapidly to the release of all new public information.

Semi-strong form efficiency contends that security prices have factored in publicly-available market and that price changes to new equilibrium levels are reflections of that information. It is considered the most practical of all EMH hypotheses but is unable to explain the context for material nonpublic information (MNPI). It concludes that neither fundamental nor technical analysis can be used to achieve superior gains and suggests that only MNPI would benefit investors seeking to earn above average returns on investments.

EMH states that at any given time and in a liquid market, security prices fully reflect all available information. This theory evolved from a 1960s PhD dissertation by U. S. economist Eugene Fama. The EMH exists in three forms: weak, semi-strong and strong, and it evaluates the influence of MNPI on market prices. EMH contends that since markets are efficient and current prices reflect all information, attempts to outperform the market are subject to chance not skill. The logic behind this is the Random Walk Theory, where all price changes reflect a random departure from previous prices. Because share prices instantly reflect all available information, then tomorrow’s prices are independent of today’s prices and will only reflect tomorrow’s news. Assuming news and price changes are unpredictable then novice and expert investor, holding a diversified portfolio, would obtain comparable returns regardless of their expertise.

The weak form of EMH assumes that the current stock prices reflect all available security market information. It contends that past price and volume data have no relationship to the direction or level of security prices. It concludes that excess returns cannot be achieved using technical analysis.

The strong form of EMH also assumes that current stock prices reflect all public and private information. It contends that non-market and inside information as well as market information are factored into security prices and that nobody has monopolistic access to relevant information. It assumes a perfect market and concludes that excess returns are impossible to achieve consistently.

EMH is influential throughout financial research, but can fall short in application. For example, the 2008 Financial Crisis called into question many theoretical market approaches for their lack of practical perspective. If all EMH assumptions had held, then the housing bubble and subsequent crash would not have occurred. EMH fails to explain market anomalies, including speculative bubbles and excess volatility. As the housing bubble peaked, funds continued to pour into subprime mortgages. Contrary to rational expectations, investors acted irrationally in favor of potential arbitrage opportunities. An efficient market would have adjusted asset prices to rational levels.

Suppose stock ABC is trading at $10, one day before it is scheduled to report earnings. A news report is published the evening before its earnings call that claims ABC's business has suffered in the last quarter due to adverse government regulation. When trading opens the next day, ABC's stock falls to $8, reflecting movement due to available public information. But the stock jumps to $11 after the call because the company reported positive results on the back of an effective cost-cutting strategy. The MNPI, in this case, is news of the cost-cutting strategy which, if available to investors, would have allowed them to profit handsomely.