Economic Modelling · Rational expectations theory and the efficient markets hypothesis

Efficient Markets Hypothesis: Definition and Weak, Semi-Strong and Strong Forms

Updated 11 October 2026 · Fact-checked

The efficient markets hypothesis (EMH) says security prices already reflect available information, so you cannot consistently earn abnormal returns from it. The weak form covers past prices, the semi-strong form adds all public information, and the strong form adds private information too. To answer a question, identify the information set.

Understand Efficient Markets Hypothesis: Definition and Forms

A market is efficient if prices quickly and fully reflect the information available. When new information arrives, prices adjust at once. So a price is the market's best estimate of value given what is known.

The idea matters because it limits what an investor can gain from analysis. If prices already contain the information, then using that information will not give returns above those justified by risk. Any abnormal return (return above what the risk taken would justify) would be a matter of luck, not skill. Price changes then come from new information, which is unpredictable by definition.

The three forms differ only in the information set that prices reflect:

Weak form: prices reflect all information in past prices and trading data (past returns, volumes). Technical analysis, which looks for patterns in past prices, should not give abnormal returns.
Semi-strong form: prices reflect all publicly available information. This includes past prices, company accounts, announcements, economic news and forecasts. Fundamental analysis using public data should not give abnormal returns. Prices adjust quickly to new public news.
Strong form: prices reflect all information, public and private. Even insiders with private information could not earn abnormal returns.

The forms are nested. Strong implies semi-strong, and semi-strong implies weak. The reverse does not follow. A market can be weak-form efficient but not semi-strong efficient.

Efficiency here is about information, not about perfect forecasts. Prices can be wrong in hindsight. The claim is that you cannot systematically exploit the information that is already in the price. Tests of efficiency are always joint tests, because you also need a model of what return is normal for the risk taken.

Key rules to remember

Weak form: Information set = past prices and trading data; Technical (chartist) analysis should not earn abnormal returns.
Semi-strong form: Information set = all public information (includes the weak-form set); Fundamental analysis on public data should not earn abnormal returns. Prices react quickly to announcements.
Strong form: Information set = all public and private information (includes the semi-strong set); Even insider information should not earn abnormal returns.
Nesting of forms: Strong ⇒ Semi-strong ⇒ Weak; If a stronger form holds, the weaker ones hold. The reverse is not true.
Abnormal return: Abnormal return = actual return − return expected for the risk taken; Efficiency says this should be zero on average when you trade on the information in the set.

How to solve Efficient Markets Hypothesis: Definition and Forms questions

Use this method for definition, classification and evidence questions on the EMH.

1Identify what information the question refers to: past prices, public information or private information.
2Match it to the form: past prices to weak, public to semi-strong, private or insider to strong.
3State the definition of that form in one sentence, naming the information set and what it implies about abnormal returns.
4Say which investment approach the form rules out: technical analysis, fundamental analysis or insider trading.
5Use the nesting rule to state what else must hold or fail. If a weaker form fails, every stronger form fails.
6Mention that tests are joint tests with a model of expected return, if the question is about evidence.
7Finish with a clear conclusion that answers the exact question asked.

Quickest way: Information-set ladder

When to use it: Multiple-choice questions and short classification questions where you have under two minutes.

Picture a ladder: past prices, then public information, then private information.
Find the highest rung the question's information sits on. That is the form being tested.
Check the nesting. Evidence that insiders profit rejects the strong form only. Evidence that chart patterns profit rejects the weak form and therefore all three.
Pick the option that matches both the information set and the strategy ruled out.

Common mistakes in Efficient Markets Hypothesis: Definition and Forms

Saying the weak form means prices are weakly related to information or that markets are inefficient.
The word weak sounds like a poor market.
Fix: Weak refers only to a small information set: past prices. A market can be weak-form efficient and fully rational.
Thinking semi-strong efficiency rules out profit from insider information.
Students mix up public and private information.
Fix: Semi-strong covers public information only. Insiders may still profit if the strong form fails.
Believing that if the strong form holds, the others may fail.
The nesting direction is reversed.
Fix: Strong includes the other two information sets, so strong implies semi-strong and weak. Failure of weak form rejects all three.
Claiming efficiency means prices are always correct or cannot fall.
Efficient is read as perfect.
Fix: Efficiency says prices reflect available information and you cannot systematically profit from it. Prices can still be wrong in hindsight.
Saying that no investor can ever outperform under the EMH, even by luck or by taking more risk.
The risk adjustment is forgotten.
Fix: Higher expected return from taking higher risk is consistent with efficiency. What is ruled out is a consistent abnormal return after allowing for risk.
Treating a failed test as proof the market is inefficient.
Students forget the joint hypothesis.
Fix: Every test also assumes a model of normal returns. A rejection may mean the market is inefficient or the model is wrong.

Worked examples

Example 1

An analyst finds that buying shares after a rise in the previous week's price gives returns above the risk-adjusted level, year after year. Which form of the EMH does this contradict? Which other forms are also contradicted? Explain.

Show the solution

The strategy uses only past price information.
Past prices are the weak-form information set, so the evidence contradicts the weak form.
By nesting, semi-strong and strong forms include the weak-form information set, so if prices do not reflect past prices, they cannot reflect all public or all information either.
So all three forms are contradicted.
Caveat: tests are joint with a model of normal returns, so the result could also reflect a poor risk adjustment.

Answer: It contradicts the weak form, and therefore also the semi-strong and strong forms. The conclusion depends on the risk model used.

Example 2

Studies show that investors trading on public company results announcements cannot earn abnormal returns after costs, but company directors who trade before announcements do. State which forms are supported and which are rejected.

Show the solution

Public announcements are part of the public information set, so the first result supports the semi-strong form.
Semi-strong efficiency includes weak-form efficiency, so the weak form is also supported.
Directors trading on information not yet public earn abnormal returns. This uses private information.
Prices therefore do not reflect all private information, so the strong form is rejected.
The result is consistent with a market that is semi-strong but not strong-form efficient.

Answer: Weak and semi-strong forms are supported. The strong form is rejected because insiders earn abnormal returns.

Exam tips

Always name the information set first. Examiners award marks for the link between the form and the information.
Use the nesting rule in any question asking what a result implies for the other forms.
In written answers, mention the joint hypothesis problem when discussing evidence.
State what each form implies for investment strategy: technical analysis, fundamental analysis and insider trading.
In multiple-choice questions, watch for options that confuse public and private information.

Practice questions from Rational expectations theory and the efficient markets hypothesis

Efficient Markets Hypothesis: Definition and Forms in other exams

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

Efficient Markets Hypothesis: Definition and Forms: frequently asked questions

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

Weak form says prices reflect all past price and trading data. Semi-strong form says prices reflect all public information, which includes past prices and also accounts, announcements and news. Semi-strong is the stronger claim.

Does the efficient markets hypothesis say prices are always correct?

No. It says prices reflect available information and that you cannot systematically earn abnormal returns from that information. Prices can turn out wrong in hindsight because new information arrives later.

Which form of the EMH is most likely to hold in practice?

The weak form is generally regarded as the easiest to support, and the strong form as the hardest, since insiders can often profit. Treat this as general reasoning, not a fixed result, and refer to the evidence you are given in the question.

Can fundamental analysis work under the semi-strong form?

Not to earn consistent abnormal returns from public data. Under the semi-strong form, public information is already in prices. Analysis might still be useful for understanding risk and value.