Financial Management · Models for the valuation of shares

Efficient Market Hypothesis and Share Price Behaviour for ACCA FM

Updated 11 October 2026 · Fact-checked

The efficient market hypothesis (EMH) says share prices reflect available information quickly and fairly. Weak form covers past price data, semi-strong adds all public information, strong form adds private information too. To answer questions, identify what information is in the price, then say whether it can be used to earn abnormal returns.

Understand Efficient Market Hypothesis and Share Price Behaviour

A share price changes when new information changes what investors expect about future cash flows or risk. A market is efficient if prices react quickly and without bias to relevant information. The price then is a fair estimate of the share's value based on that information.

The hypothesis has three forms. They differ in the information assumed to be already in the price. Each form includes the one before it.

Weak form: the price reflects all past price and trading data. Studying charts and past price patterns (technical analysis) cannot give consistent abnormal returns. Prices follow a random walk, because new information arrives unpredictably.
Semi-strong form: the price reflects all publicly available information. This includes past prices, annual reports, announcements and news. Neither technical analysis nor fundamental analysis of public data gives consistent abnormal returns. The price adjusts almost instantly when news is released.
Strong form: the price reflects all information, public and private, including insider information. Even insiders cannot make abnormal gains.

In practice, major stock markets are generally regarded as semi-strong form efficient, with the evidence being mixed. Strong form efficiency is not believed to hold, because insider dealing can earn abnormal gains (which is why it is illegal in many countries).

For a financial manager the point is practical. If the market is efficient, the share price is a fair measure of value, so managers can use it to judge the effect of decisions on shareholder wealth. The timing of a share issue is not worth trying to game. Creative accounting that changes only reported profit, not cash flows, should not fool investors. Financing choices that add no real value will not raise the price. Share prices will react to genuine news, so good communication matters.

Key rules to remember

Weak form: Price = f(past prices and trading data); Technical (chartist) analysis is of no use. Insiders and fundamental analysts could still gain.
Semi-strong form: Price = f(all public information); Includes weak form. Fundamental analysis of public data cannot beat the market consistently. Insider information can.
Strong form: Price = f(all public and private information); Includes semi-strong form. No one can earn consistent abnormal returns.
Abnormal return: Abnormal return = actual return − return expected for the risk taken; EMH says abnormal returns cannot be earned consistently using the information already in the price.

How to solve Efficient Market Hypothesis and Share Price Behaviour questions

Use this method for any EMH question, whether objective test or written.

1Identify what information the question refers to: past prices, public news, or private information.
2Match it to the form of efficiency: past prices = weak, public = semi-strong, private = strong.
3Remember each form includes the lower ones. Strong implies semi-strong, which implies weak.
4State what the form implies: whether technical analysis, fundamental analysis or insider information can earn abnormal returns.
5If it is a scenario, check how the price actually behaved: did it move on the announcement date, before it, or slowly afterwards?
6Draw the conclusion for the financial manager: is the price a fair value, and does the decision affect shareholder wealth?
7In a written answer, add one limitation or piece of evidence for and against efficiency if marks allow.

Quickest way: Information ladder

When to use it: Use for Section A and Section B objective questions that ask which form of efficiency is described or violated.

Ask: what information could the investor use to gain? Past prices, public news or inside information.
If gaining from past prices is impossible but public or insider information can help, the market is weak form only.
If public information gives no gain but insider information does, it is semi-strong form.
If nobody gains, even with insiders, it is strong form.
Check the answer options for the exact wording. All-or-nothing marking means one wrong word loses the mark.

Common mistakes in Efficient Market Hypothesis and Share Price Behaviour

Saying weak form means prices are weak or inaccurate.
The word 'weak' is read as a quality judgement.
Fix: Weak refers to the small set of information in the price: past prices only.
Treating the three forms as separate, not cumulative.
Students memorise the three definitions as a list.
Fix: Each form includes the previous one. If a market is semi-strong, it is also weak form.
Claiming that in an efficient market share prices never change or are always correct.
Confusing efficiency with stability or perfect foresight.
Fix: Prices change whenever new information arrives. Efficient means they react quickly and without bias, not that they are certain.
Saying that fundamental analysis works in a semi-strong market.
Mixing up technical and fundamental analysis.
Fix: Technical analysis uses past prices. Fundamental analysis uses public financial data. Semi-strong form says neither earns consistent abnormal returns.
Stating that real markets are strong form efficient.
Overstating the theory.
Fix: Evidence suggests major markets are broadly semi-strong, with exceptions. Insider dealing gains show strong form does not hold.
Listing implications for managers without linking them to shareholder wealth.
Memorised bullet points with no reasoning.
Fix: Connect each point to the price: for example, if the price is fair, a project with positive NPV should raise it, whereas cosmetic accounting changes should not.

Worked examples

Example 1

Shares in Kestrel Co rose sharply the day after it published unexpectedly high profits. An investor who bought shares only after reading the profit announcement made no abnormal return. An investor with advance knowledge of the profits did make an abnormal gain. Which form of efficiency does this suggest, and why?

Show the solution

Identify the information: the profit announcement is public information once published.
The investor who traded after publication gained nothing, so public information is already in the price. This means the market is at least semi-strong form.
The investor with advance knowledge, who held private information, did gain. So private information is not in the price.
Therefore the market is not strong form efficient.

Answer: The market is semi-strong form efficient but not strong form. Public information is reflected in the price quickly, but private information is not.

Example 2

Explain to the board of Delta plc, whose shares trade on a semi-strong form efficient market, two implications for its financial managers.

Show the solution

State the assumption: all public information is quickly reflected in Delta's share price, so the price is a fair value given that information.
Implication 1: the share price is a good measure of the effect of decisions on shareholder wealth. Accepting positive NPV projects should raise the price once the market learns of them, so managers can focus on real value.
Implication 2: changes in accounting policy that alter reported profit but not cash flows should not raise the price, as investors can see through them. Creative accounting is pointless.
Add a further point if needed: timing a share issue to catch a high price is not worthwhile, because the price is already fair, and good communication of genuine news matters because the price reacts to it.
Caveat: semi-strong form does not rule out gains from inside information, so directors must not use it.

Answer: The share price is a fair measure of value, so decisions should be judged on their effect on shareholder wealth. Cosmetic accounting changes will not fool the market, and trying to time share issues does not add value. Genuine, public news does move the price.

Exam tips

Be ready to name the form from a short description. Look for the key words: past prices, public information, insider information.
In written answers, always link EMH to a decision for the financial manager, such as investment, financing, dividend policy or reporting.
Do not claim a form of efficiency is proved. Say evidence suggests markets are broadly semi-strong, with anomalies.
In objective questions with several statements, check each one against the cumulative rule before choosing, as marks are all or nothing.

Practice questions from Models for the valuation of shares

Efficient Market Hypothesis and Share Price Behaviour in other exams

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

Efficient Market Hypothesis and Share Price Behaviour: frequently asked questions

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

Weak form means the price reflects only past price and trading data. Semi-strong form means it reflects all public information as well, such as financial statements and announcements. So technical analysis fails in both, but fundamental analysis of public data fails only in semi-strong.

Which form of efficiency do real markets show?

Evidence suggests major markets are broadly semi-strong form efficient, although there are exceptions. Strong form is not believed to hold, because people with inside information can gain.

What does the efficient market hypothesis mean for financial managers?

The share price is a fair reflection of public information, so it can be used to judge the effect of decisions on shareholder wealth. Managers cannot expect to fool the market with cosmetic accounting or gain by timing issues. Genuine news and clear communication are what move the price.

Does an efficient market mean share prices never fall?

No. Prices move whenever new information arrives, and bad news can reduce the price. Efficiency means prices adjust quickly and without bias to that information.