Chapter 08 – Stock Markets 6th Edition
If one can use historical news, or historical price and volume information to
consistently predict future stock price changes then the markets are not weak form
efficient.
If one can use any publicly available information, including forecasts based on
publicly available information, to consistently predict future stock price changes then
the markets are not semi-strong form efficient.
If one can use any information, including ‘inside’ information to consistently predict
future stock price changes then the markets are not strong form efficient.
The stock markets appear to exhibit weak form efficiency and with some glaring
exceptions, appear to be semi-strong efficient as well. The markets are not strong form
efficient. If they are the government owes some major apologies to people such as
Martha Stewart, Dr. Waksal, Michael Milken, Ivan Boesky and others who were arrested
for insider trading.1
Teaching Tip: The markets can only be strong form efficient if a) market prices are
irrational or b) insiders do not really have useful information so the question is moot, or
c) general market volatility is so large that inside information is not useful It is the
actions of profit seeking, non-collusive traders that bring about market efficiency. Why
would one expect prices to reflect information that no trader has access to before the
trades makes the inside information public? By what mechanism would this information
be reflected in prices?
A more strict definition of efficiency is that stock prices follow a random walk, akin to
the path an inebriated person would take staggering across a field. The person’s last step
cannot tell us anything about their next step because the choice of direction of the next
step is random. This implies zero serial correlation of returns. Stock prices do not
strictly conform to a random walk, but deviations are generally not large enough to allow
abnormal trading profits after consideration of trading costs.
c. Stock Market Regulations
The SEC oversees the exchanges and The Financial Industry Regulatory Authority
(FINRA) is the regulator for all U.S. securities firms. FINRA was created by the 2007
merger of the NASD and the regulatory arm of the NYSE. FINRA oversees registering
and educating brokers and dealers, examining securities firms, promulgating rules,
enforcing federal securities laws and conducting dispute arbitration. The SEC has been
heavily criticized for failing to uncover the Madoff fraud. This led to many personnel
changes and supposedly a more adversarial culture at the agency.
One of the functions of the SEC is to limit insider trading and stock price manipulation
and promote investor confidence in the markets. In October 2000 the SEC adopted
Regulation Fair Disclosure to stop firms from selectively disclosing information to only
1 One could still argue they acted unfairly, but if the markets are strong form efficient
there would be no damages resulting from their actions unless outright market
manipulation was involved.
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