Efficient Market Theory
Module 9 · Investor Behavior · Lesson 3
What Does Market Efficiency Mean?
The efficient-market hypothesis asks how quickly prices reflect available information. It is a framework for thinking about the difficulty of gaining an edge, not a claim that every price is always correct.
Quick Answer: What is an efficient market?
In Fama’s definition of an efficient market, prices fully reflect all available information. Eugene Fama’s research distinguishes tests based on past prices, public information, and private information. The idea does not mean that a future price is known, that prices never deviate from a reasonable estimate, or that investors cannot lose money.
What are the three information sets?
- Weak-form tests ask whether past price information alone can reliably predict risk-adjusted future returns.
- Semi-strong-form tests ask how prices respond to publicly available announcements and disclosures.
- Strong-form tests ask whether even private information is reflected in price. This is a much stronger claim and should not be assumed to describe actual markets.
These are ways of framing empirical tests, not three labels that permanently classify a particular stock.
Why does the idea matter to an investor?
If widely available information is incorporated quickly into prices, buying solely because of a familiar headline may offer no advantage. Active investing tries to make decisions that outperform an appropriate benchmark; passive index funds generally aim to track one before fees. FINRA explains that either approach has trade-offs, costs, and risks. A research process should account for fees, taxes, diversification, and the possibility that an apparent pattern happened by chance.
What does the hypothesis not prove?
It does not prove that all investors are rational or that every share is fairly valued at every instant. It also does not prove that a chart pattern or stock picker will succeed. Claims about an edge require a defined benchmark, an appropriate risk comparison, costs, and evidence beyond a few selected examples.
How can you read a market-efficiency claim carefully?
- Ask which information set and time period the claim concerns.
- Check whether the example uses information that was known before the price move.
- Compare performance after fees with a suitable benchmark and risk level.
- Recognize that a single announcement or trade cannot settle a broad market theory.
A dated test: US active equity funds over 15 years
| Fund category | Comparison index | Underperforming |
|---|---|---|
| All domestic | S&P Composite 1500 | 93.15% |
| All large-cap | S&P500 | 89.93% |
| All mid-cap | S&P MidCap 400 | 84.49% |
| All small-cap | S&P SmallCap 600 | 89.90% |
- Choose a category and its corresponding benchmark before comparing results.
- For large-cap funds, read 89.93% as about 90 out of 100 funds in the reported starting opportunity set underperforming the S&P 500 over this period.
- Check the method: SPIVA accounts for merged or liquidated funds, and fund returns are net of fees excluding sales loads.
These historical category results make consistent outperformance a demanding benchmark. They do not prove every price is correct, establish the cause of underperformance, or predict the next 15 years. An index is not directly investable; a fund tracking it also has costs.
Source: S&P Dow Jones Indices, SPIVA US Year-End 2025, Report 1 a and methodology.
Key takeaways
- Market efficiency is a hypothesis about information and prices, not a promise of a correct price at every moment.
- Compare a strategy with a suitable benchmark, risk level, time period, and costs.
- Historical fund-category results do not predict a particular fund’s future performance.
Check your understanding
1. Which question fits a semi-strong-form test?
A. How prices respond to public disclosures. B. Whether all prices never move. C. Whether returns are guaranteed.
Show answer and feedback
A. Semi-strong tests concern publicly available information and the speed of price response.
2. Does market efficiency mean a stock cannot lose value?
A. Yes. B. No. C. Only in an index fund.
Show answer and feedback
B. Prices can change as information and expectations change; risk remains.
3. What is needed to judge an active strategy fairly?
A. Its best single trade. B. A headline about one stock. C. A relevant benchmark, risk, and costs.
Show answer and feedback
C. A fair comparison considers outcomes, risk, and expenses over a useful period.
Why question: Why might a correct public earnings headline offer little trading advantage by the time you read it?
Show a sample explanation
Other investors may have seen and acted on the same public information, moving the price. The headline may also leave out expectations already reflected in the earlier price.
Try it: separate a filing from a price move
Open the SEC EDGAR search prefilled for Apple and note the date and time of a recent 8-K or earnings-related filing. Then open the stock chart with AAPL loaded and identify the nearby daily move. Write down what the filing actually disclosed and one reason the price move cannot be attributed to that document alone. A daily chart does not establish the exact reaction time.
Where can you check the sources?
- Fama (1970): Efficient Capital Markets: A Review of Theory and Empirical Work; the definition is quoted on the first page of Bai, Philippon and Savov’s 2016 research.
- Eugene Fama: Efficient Capital Markets II, Journal of Finance
- FINRA: Active vs. Passive Investing
- SEC Investor.gov: Index Funds
- SEC Investor.gov: Using EDGAR to Research Investments
End of Lesson · Continue to Cognitive Biases
AI responses are for educational purposes. Always verify with official course materials.
This lesson teaches a concept. It is general education, not advice about your money. Before you act on anything you learn here, speak to someone licensed.
AI responses are for educational purposes. Always verify with official course materials.
This lesson teaches a concept. It is general education, not advice about your money. Before you act on anything you learn here, speak to someone licensed.
This lesson teaches a concept. It is general education, not advice about your money. Before you act on anything you learn here, speak to someone licensed.
