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Market Efficiency & Stylesintermediate

The Efficient Market Hypothesis

The Efficient Market Hypothesis (EMH) holds that security prices already reflect available information, so prices adjust quickly and without bias as news arrives. Fama framed three nested forms by the information set considered. The weak form says prices embed all past price and volume data, which makes technical analysis unable to earn abnormal returns. The semi-strong form says prices embed all public information, including financial statements and announcements, which undercuts fundamental analysis of public data. The strong form says prices embed even private, inside information, which essentially no real market satisfies.

Why it matters

If a piece of information helps predict a price, traders chase that edge until it is bid away, so the information ends up baked into the current price. The stronger the form, the wider the information set already priced. Weak form prices in the chart, semi-strong form prices in everything public, and strong form prices in the secrets too. Most evidence places real markets somewhere between the weak and semi-strong forms, which is why beating the market consistently is so hard rather than impossible.

Formulas

Fair-game property of returns
E ⁣(rt+1∣Ωt)=E(rt+1)E\!\left(r_{t+1} \mid \Omega_t\right) = E(r_{t+1})
Given the information set Ωt\Omega_t already reflected in price, the best forecast of the next return is just its normal expected value. No element of Ωt\Omega_t improves the forecast, so abnormal profits from that information are not available.

Worked examples

Scenario

A firm reports earnings far above forecast. Under the semi-strong form, when can a trader profit from buying after reading the headline?

Solution

Not after the announcement is public. The semi-strong form says the price jumps to its new level the moment the surprise becomes public, so a trader reading the headline is already too late. Studies of post-announcement returns show most of the adjustment happens within minutes. Any reliable profit would require acting on the information before it became public, which is the strong-form case and is also illegal as insider trading.

Scenario

An analyst claims a chart pattern signals a coming rally. Which EMH form does this challenge?

Solution

The weak form. A tradeable pattern in past prices would mean past price data are not fully reflected in the current price, contradicting the weak form. Because price history is the cheapest and most widely studied information, the weak form is the one most strongly supported by evidence, which makes such claims hard to sustain after trading costs.

Common mistakes

  • ✗The EMH says prices are always right. It says prices reflect available information without systematic bias, not that they equal true value. Prices can be wrong, but the errors are not predictable from the information already public.
  • ✗Market efficiency means no one can ever beat the market. Some investors beat it in any period by chance, and a few may exploit information faster. Efficiency claims that beating it reliably, after risk and costs, is very hard, not that it never happens.
  • ✗A market crash disproves the EMH. Large moves can reflect rapid repricing as information and risk premia change. A crash only contradicts efficiency if it was predictable in advance from public information, which is the harder claim to establish.
  • ✗The strong form is the realistic case for most markets. Almost no market is strong-form efficient, since insiders demonstrably profit. Real markets are usually argued to sit near the weak-to-semi-strong boundary.

Revision bullets

  • •EMH: prices reflect available information and adjust quickly, without bias
  • •Weak form prices in past price and volume, so technical analysis cannot earn abnormal returns
  • •Semi-strong form prices in all public information, undercutting fundamental analysis of public data
  • •Strong form prices in private information too, a case almost no real market meets
  • •Efficiency is about no predictable abnormal profit, not prices being correct

Quick check

Which form of the EMH directly implies that technical analysis cannot earn consistent abnormal returns?

Under the semi-strong form, what should happen to a stock price when a genuine earnings surprise is announced publicly?

Connected topics

Active vs PassiveBehavioral FinanceFundamentalTechnical Analysis

Sources

  1. Brailsford, Heaney & Bilson (2015), Ch. 13
    Brailsford, T., Heaney, R., & Bilson, C. Investments: Concepts and Applications. 5th ed. Cengage Learning Australia, 2015.
    Presents the three forms of the Efficient Market Hypothesis and the information set each implies.
  2. Bodie, Kane & Marcus (2021), Ch. 11
    Bodie, Z., Kane, A., & Marcus, A. J. Investments. 12th ed. McGraw-Hill Education, 2021.
    Reference treatment of the EMH, the fair-game model, and the evidence on each form.
How to cite this page
Dr. Phil's Quant Lab. (2026). The Efficient Market Hypothesis. Derivatives Atlas. https://phucnguyenvan.com/concept/im-emh
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