Market Efficiency and Return Predictability

Market Efficiency and Return Predictability explains what it means for prices to reflect information and what researchers can, and cannot, infer from patterns in historical returns.

Use this branch when evaluating an efficient-market claim, a return anomaly, an event study, a price-predictability test, or an argument for active or passive implementation. These concepts are empirical frameworks, not declarations that every market price is correct or that a discovered pattern is tradable.

What This Branch Covers

How the Concepts Differ

Market efficiency is an economic hypothesis about information and prices. A random walk is a statistical model for price changes. They are related, but one does not mechanically prove the other.

An efficient market can have time-varying expected returns if compensation for risk changes. A statistical rejection of a strict random walk can reflect market microstructure, changing risk, or a pattern too small to exploit after costs. Likewise, failure to reject a random walk does not prove that all public or private information is already reflected in price.

Evidence to Check

• the information set available at the decision time
• the expected-return or asset-pricing model used as the benchmark
• sample period, market, security universe, and survivorship rules
• event timestamps, announcement leakage, and confounding news
• bid-ask spreads, commissions, market impact, shorting, financing, and taxes
• out-of-sample performance and robustness across definitions
• multiple-testing, selection, publication, and look-ahead bias
• capacity, turnover, liquidity, and whether results survive implementation

Common Mistakes

• Interpreting market efficiency as a claim that prices never deviate from fundamental value.
• Treating a backtested anomaly as a guaranteed or risk-free return.
• Ignoring that every efficiency test also tests a benchmark model of expected return.
• Equating a random walk with equal probabilities of an up or down move.
• Comparing gross research results with net investable performance.

This branch is educational and does not recommend active trading, passive investing, a security, a fund, or a portfolio strategy.

In this section

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