What is Market Efficiency Hypothesis?
The Efficient Market Hypothesis (EMH) is a foundational concept in finance stating that asset prices instantly reflect all available information — making it impossible to consistently earn abnormal returns through research or timing. EMH exists in three forms, each with different implications for investors.
The EMH states that asset prices incorporate all public and private information, so investors cannot predictably beat the market by trading on known data.
- 1↓Weak Form EfficiencyPrices reflect all past trading data (history, volume). Technical analysis fails.
- 2↓Semi-Strong FormPrices reflect all public info (news, reports, earnings). Fundamental analysis fails.
- 3Strong Form EfficiencyPrices reflect ALL info, public + private (insider data). No analysis works.
Step-by-step worked examples
A company announces 15% profit growth (public news). In efficient markets, what happens?
Weak form: History can't predict this announcement. Semi-strong: Price jumps instantly; the news is in the price. No investor can profit by trading after the announcement — it's already priced in.
An analyst finds a 'pattern' in past stock prices suggesting future rises. Will it work?
In weak-form efficient markets, historical patterns have no predictive power. Past price movements are already reflected in current prices. The analyst cannot consistently beat the market using technical analysis.
An insider knows about a merger before it's public. Can they profit?
In strong-form efficiency, even insiders cannot profit — all info is in the price. But strong-form EMH is rare (insider trading laws exist for a reason). In reality, insiders CAN profit (violating strong-form EMH).
Flashcards
Quick quiz
Q1.EMH implies that technical analysis…
Q2.If EMH is semi-strong, what makes money?
Q3.A stock drops 5% on negative earnings news. In efficient markets…
Q4.Why is insider trading illegal if EMH is true?
The full card deck, worked steps and AI-tutor support for “What is Market Efficiency Hypothesis?” are in Notek — study by hand before your exam.
Common mistakes
If markets are efficient, you can pick stocks with research. — Correct: EMH says public research is already priced in — exceptional research may help, but consistent outperformance is rare.
EMH means all investors are rational. — Correct: EMH means market prices reflect info, not that individuals are rational — emotions balance out in aggregate.
Strong-form EMH is definitely true. — Correct: Strong-form EMH is weakest — insider trading laws and insider profits suggest it doesn't hold.
If you find a pattern, EMH is false. — Correct: One anomaly doesn't disprove EMH — if a pattern is exploitable, traders use it and it disappears (self-correcting).
FAQ
What is market efficiency hypothesis?
EMH states that asset prices instantly incorporate all available information, making it impossible to predictably beat the market through analysis or timing.
Can you beat the market if EMH is true?
Not consistently — but markets may have temporary inefficiencies. Most actively managed funds underperform index funds, supporting weak EMH.
Which form of EMH is most realistic?
Semi-strong form is most supported by evidence — public news is quickly priced, but some private-info traders profit (contradicting strong-form).
Why do hedge funds exist if markets are efficient?
Hedge funds seek market inefficiencies and use leverage/derivatives. Few consistently outperform after fees — supporting EMH.