CFA Level I Exam · Equity Issuance and Trading

Market Efficiency and Anomalies for CFA Level I

Updated 7 October 2026 · Fact-checked

An efficient market is one where prices quickly reflect available information. Weak form reflects past trading data, semi-strong adds all public information, and strong adds private information too. To answer questions, identify what information a strategy uses, then decide which form its success would contradict.

Understand Market Efficiency and Anomalies

A market is efficient when security prices fully and quickly reflect the information available to investors. If that is true, you cannot reliably earn returns above those justified by risk, after costs, by trading on that information.

The efficient market hypothesis has three forms. Weak form: prices reflect all past market data, such as prices and volumes. Technical analysis should not earn excess returns. Semi-strong form: prices reflect all public information, including financial statements and news. Fundamental analysis should not earn excess returns either. Strong form: prices reflect all information, public and private. Even insiders could not earn excess returns.

The forms are nested. Strong form efficiency implies semi-strong and weak form efficiency, because the strong form information set includes the other two. So if a market is semi-strong efficient, it is also weak form efficient. It follows that rejecting a lower form rejects all higher forms. Evidence of insiders profiting consistently contradicts only the strong form. Evidence that public news can be exploited contradicts semi-strong and therefore strong.

Do not mix up two ideas. Informational efficiency is about how well prices reflect information. Operational efficiency (also called internal efficiency) is about how cheaply and smoothly trades happen: low transaction costs, tight bid-ask spreads, and fast execution. A well-functioning financial system also has liquidity, complete markets (investors can save, borrow, hedge and invest as they need), and informational efficiency, with prices that reflect fundamentals so capital flows to its best uses.

An anomaly is a pattern that seems to contradict market efficiency, such as the January effect, momentum, size or value effects, and the post-earnings-announcement drift. Treat anomalies with care. Many vanish after transaction costs, may come from data mining or a flawed risk model, and often disappear once discovered. Evidence can also depend on the model of expected returns used, so a test of efficiency is always a joint test with that model.

Key formulas to remember

Weak form: Prices reflect all past market data (prices, volume); Technical analysis should not earn excess returns. Fundamental analysis and insider information might.
Semi-strong form: Prices reflect all public information (past data plus public news and filings); Fundamental analysis should not earn excess returns. Insider information might.
Strong form: Prices reflect all information, public and private; No one earns excess returns, including insiders. Evidence that insiders can earn abnormal returns generally rejects this form.
Nesting of forms: Strong ⇒ Semi-strong ⇒ Weak; Strong form efficiency implies semi-strong and weak form efficiency. So rejecting a lower form rejects all higher forms: rejecting weak form rejects all three. Rejecting strong form is consistent with weak and semi-strong forms still holding, but it does not prove them.
Informational vs operational efficiency: Informational: prices reflect information. Operational: low trading costs and efficient execution; A market can be operationally efficient yet informationally inefficient, and the reverse.

How to solve Market Efficiency and Anomalies questions

Use this routine on any question about efficiency forms, anomalies or market quality.

1Underline the information the strategy or evidence uses: past prices and volume, public news or filings, or private information.
2Match that information to the form: past market data is weak, public is semi-strong, private is strong.
3Ask what the question wants: which form is contradicted, which analysis is useless, or what the evidence implies.
4Apply nesting. Evidence against a lower form also rejects the higher forms.
5If the question is about costs, spreads or execution, it is operational efficiency. If it is about price reflecting information, it is informational efficiency.
6For anomalies, check for transaction costs, data mining, risk-model problems and whether the pattern persists. Do not accept it as proof of inefficiency.
7Eliminate the two options that name the wrong information set, then confirm the last one.

Quickest way: Information-set shortcut

When to use it: Use when the stem describes a strategy and asks which form it violates or which analysis fails.

Ask: what data does the strategy use?
Past prices or volume: weak form. Public news or ratios: semi-strong. Inside information: strong.
Pick the form that matches the strategy's information set, meaning the lowest form whose information set the strategy uses, if the question asks which form is violated and the strategy works.
Remember that technical analysis is tested by weak form, fundamental analysis by semi-strong form.
Confirm with nesting, then answer in under 60 seconds.

Common mistakes in Market Efficiency and Anomalies

Saying weak form efficiency means prices cannot be predicted by any information.
The word 'weak' is read as 'no efficiency'. Students forget each form covers a specific information set.
Fix: Weak form only says past market data has no predictive value. Public or private information might still help.
Concluding that a profitable insider trading record rejects semi-strong efficiency.
Students forget that implication runs one way: strong form implies the lower forms, but rejecting strong form does not reject them.
Fix: Insider profits contradict only strong form. To reject semi-strong, a strategy must succeed using public information.
Treating anomalies as proof the market is inefficient.
A pattern in historical data looks like a free profit.
Fix: Check transaction costs, data mining, risk adjustment and persistence. Efficiency tests are joint tests with an expected-return model.
Confusing operational and informational efficiency.
Both use the word 'efficient' and both relate to market quality.
Fix: Operational is about costs, spreads and speed of trading. Informational is about prices reflecting information.
Assuming efficient markets mean analysts have no role.
Students read 'cannot beat the market' as 'analysis is pointless'.
Fix: Analysts' work is what makes prices efficient. Efficiency means mispricings are small and quickly corrected, and the benefit of analysis must be weighed against its cost.

Worked examples

Example 1

An analyst finds that buying stocks after they rise on heavy volume produces consistent excess returns after costs over many years. Which form of market efficiency does this evidence contradict most directly? A. Weak form B. Semi-strong form C. Strong form

Show the solution

The strategy uses past prices and trading volume, which are past market data.
Past market data is the weak form information set.
Consistent excess returns from it contradict weak form.
By nesting, semi-strong and strong forms are also contradicted, but the most direct contradiction is the lowest form whose information set the strategy uses, which is weak form.
Options B and C name higher forms whose information sets go beyond what the strategy uses, so they are not the most direct contradiction and are eliminated.

Answer: A. Weak form

Example 2

A researcher shows that corporate insiders earn abnormal returns, but investors who trade only on public filings and news do not. Which is the most consistent conclusion? A. The market is weak form inefficient B. The market is semi-strong efficient but not strong form efficient C. The market is strong form efficient

Show the solution

Investors using public information earn no abnormal returns, so the market is semi-strong efficient.
Semi-strong efficiency implies weak form efficiency, so weak form is not rejected. This eliminates A.
Insiders earn abnormal returns using private information, which contradicts strong form. This eliminates C.
The market is therefore semi-strong but not strong form efficient.

Answer: B. The market is semi-strong efficient but not strong form efficient

Exam tips

Always name the information set first. It decides the form in almost every question.
Remember nesting: rejecting a lower form rejects the higher ones, not the other way round.
Watch for wording about cost, spreads and speed. That points to operational efficiency, not informational.
Anomaly questions usually reward caution: transaction costs, data mining and risk adjustment explain apparent excess returns.
When the strategy uses only public data, the form it most directly contradicts is semi-strong (or weak if it uses only past prices), not strong form alone.

Practice questions from Equity Issuance and Trading

Market Efficiency and Anomalies in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Market Efficiency and Anomalies: frequently asked questions

What is the difference between weak, semi-strong and strong form efficiency?

They differ in the information prices reflect. Weak form reflects past market data, semi-strong form adds all public information, and strong form adds private information. Each form includes the one before it.

What is the difference between operational and informational efficiency?

Operational efficiency concerns how cheaply and quickly trades are executed, shown by low costs and tight spreads. Informational efficiency concerns how fully and quickly prices reflect available information.

What are examples of market anomalies?

Common examples are the January effect, momentum, the size effect, the value effect and post-earnings-announcement drift. They look like exceptions to efficiency but may disappear after costs or reflect data mining or unmodelled risk.

What are the characteristics of a well-functioning financial system?

It lets people save, borrow, raise equity, manage risk, trade assets and exchange currencies. It has complete markets, liquidity, and operational and informational efficiency, so capital is allocated to its best use.