What it means
The efficient market hypothesis comes in three strengths. The weak form says prices reflect all past trading data, so charts cannot predict the future.
The semi-strong form says prices also reflect all public information, such as reports and news, so analysing public data cannot give an edge. The strong form goes further and says prices reflect everything, public or private.
In that world, a director who knew about an unannounced takeover could not profit from it, because the price would already include the news. This is a very strong claim and is the easiest of the three to challenge.
The evidence suggests it does not hold in practice. Insider trading laws exist because insiders have often made large profits from non-public knowledge, which would not be possible if prices already reflected it.
Studies of lawful insider trades, which are reported publicly, have also tended to find that insiders earn returns above the market. Even so, the idea is useful as a benchmark.
It tells investors that the more information is shared openly, the less advantage anyone holds, and it explains why disclosure rules matter. Company finance teams who manage inside information, such as pending results or mergers, rely on the logic that the news will move the price once it is released.
For non-specialists, the key point is to separate the strong form from the others. A market can be efficient enough that public analysis rarely pays, and still be far from strong-form efficient, because insiders know things the public does not.
Regulators treat the gap as a reason for fair-disclosure rules and trading restrictions. Testing the idea means measuring abnormal returns, which are the gains beyond what would be expected for the risk taken.
If a group such as company insiders consistently earns positive abnormal returns, the strong form is rejected for that group.
In practice
Real-world examples.
Example
A regulator reviews trades in a company's shares in the week before a surprise takeover bid and finds that purchases by a small group of people produced unusually high gains. This pattern would not exist if the strong form held. The regulator opens an inquiry into possible insider dealing.
Example
A finance professor teaching a course uses the strong form as the extreme case. She asks students why company directors are required to publish their share dealings. The answer is that these dealings carry information the market does not already have.
Example
A company's legal team sets a blackout period during which executives cannot trade shares before results are released. The rule exists because private information is valuable, which is the opposite of the strong form assumption.
Formula
Calculation
Abnormal return = actual return - expected return
Suppose a researcher studies a director's share purchases before an unannounced good-news event. The shares are expected, based on the market and the company's risk, to return 8% over the period. The shares actually return 14%. The abnormal return is 14% - 8% = 6%. If many such cases show consistent positive abnormal returns, the strong form is not supported; on an investment of $100,000, this represents an unexpected gain of $6,000.Case study
Seen in the real world.
Marlowe Pharma is an illustrative, fictional drug developer. Its finance director was asked by the board why employees were barred from trading in the company's shares during a clinical trial. A junior colleague argued that the market was efficient, so inside knowledge should not matter.
The finance director explained that the argument only worked under the strong form, which almost nobody believes. She showed that, in earlier trials, the share price had jumped by 40% when positive results were announced, a move that could not have happened if the price had already included the result.
The board kept the trading ban and added training on handling inside information. The illustrative lesson is that the strong form is useful as a thought experiment, and its failure is the reason rules on insider trading are needed.
Watch out
Common mistakes.
- Assuming that markets are efficient in the strong sense because they react quickly to public news, which is a weaker claim.
- Confusing the strong form with the semi-strong form, which only covers public information.
- Believing that the strong form means insider trading is harmless, when the evidence is that it is both profitable and harmful to other investors.
Questions
People also ask.
Is the strong form of market efficiency true?
Most evidence says no, because insiders have been shown to earn above-market returns using non-public information.
What is the difference between the three forms?
Weak form covers past prices, semi-strong form adds public information, and strong form adds private information as well.
Why does the strong form matter if it is not true?
It sets the outer limit of efficiency and explains why disclosure and insider trading rules protect other investors.
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