Business Finance · Key principles of finance

Efficient Markets and Arbitrage: Weak, Semi-Strong and Strong Form

Updated 11 October 2026 · Fact-checked

The efficient market hypothesis says security prices already reflect available information. Weak form covers past prices, semi-strong covers all public information, strong covers all information including private. Arbitrage is a riskless profit from zero net investment. In an arbitrage-free market, assets with identical cash flows must have the same price.

Understand Efficient Markets and Arbitrage

A market is efficient if prices reflect information quickly and fully. If so, you cannot consistently earn returns above those justified by risk using that information. Prices still move. They move when new information arrives, and new information is unpredictable, so price changes look random.

The efficient market hypothesis (EMH) has three forms, defined by the information set reflected in prices.

Weak form: prices reflect all past price and trading data. Technical analysis (chart patterns) cannot give consistent excess returns.
Semi-strong form: prices reflect all publicly available information, including past prices, accounts and announcements. Fundamental analysis on public data cannot give consistent excess returns. Prices adjust quickly to new public news.
Strong form: prices reflect all information, public and private. Even insiders cannot earn excess returns.

Each form includes the one before it. If the strong form holds, so do the semi-strong and weak forms. The reverse is not true. Evidence generally supports the weak form more than the semi-strong, and the strong form is widely doubted because insider trading can be profitable (and is illegal for this reason).

An arbitrage is a strategy that needs no net outlay (or gives a profit now), has no chance of a loss, and has a positive chance of a gain. The no-arbitrage principle assumes such opportunities do not last, because traders would exploit them and move prices until they vanish. This gives the law of one price: two assets or portfolios with identical cash flows in all states must have the same price now.

No-arbitrage is the basis of pricing for forwards, bonds, options and the CAPM-free pricing methods you meet in CM2. For Business Finance, the link to valuation is simple. If markets are efficient, the market price is a fair estimate of value, so a project or share cannot be expected to have a positive NPV from public information alone. Managers should not time issues or mistime share prices, and accounting changes that do not alter cash flows should not change value.

Key rules to remember

Law of one price: If two portfolios give identical cash flows in every state, then price A = price B; If prices differ, buy the cheaper and sell the dearer to lock in a riskless profit.
Arbitrage condition: Initial cost ≤ 0, payoff ≥ 0 in all states, payoff > 0 in at least one state with positive probability; A strategy must satisfy all of these. Mere high expected profit is not arbitrage.
Forward price (no income, continuous compounding): F = S × e^(δT); S is the spot price, δ the risk-free force of interest, T the time to delivery. Use F = S × (1 + i)^T with annual compounding.
Forward price with known income: F = (S − PV of income) × (1 + i)^T; Subtract the present value of income paid before delivery. Valid under no-arbitrage with no transaction costs.
Information sets in EMH: Weak ⊂ Semi-strong ⊂ Strong; Each stronger form includes all the information of the weaker one.

How to solve Efficient Markets and Arbitrage questions

Exam questions on this topic are either descriptive (forms of EMH and implications) or numerical (find an arbitrage or a no-arbitrage price). Use this method.

1Identify the type of question: classify an EMH form, discuss implications, or test for arbitrage.
2For EMH questions, name the information set (past prices, public, or all) that the evidence or strategy uses.
3Decide which form the evidence breaks. A strategy using only past prices tests the weak form. One using public accounts tests semi-strong. One using insider information tests strong.
4For arbitrage questions, write the cash flows of both positions at time 0 and at the future date.
5Check the conditions: no net cost, no possible loss, a positive chance of gain. If prices differ for identical cash flows, buy the cheap one and sell the dear one.
6Compute the no-arbitrage price from the replicating position, stating assumptions such as no transaction costs, no taxes and borrowing and lending at the same rate.
7State the conclusion in words, linking it to valuation: market price as fair value, or the pressure that removes the mispricing.

Quickest way: Information-set test and cost-today test

When to use it: Use in multiple-choice questions where you must classify an EMH form or spot an arbitrage quickly.

Ask what information the strategy uses. Past prices means weak, public news means semi-strong, inside information means strong.
Remember that a failure of a weaker form means all stronger forms also fail.
For arbitrage, compute the cost of the replicating portfolio and compare it with the quoted price.
If the quoted price is higher, sell it and buy the replicating portfolio. If lower, do the reverse.
Profit today equals the price difference, accumulated if needed. Check that the future cash flows cancel.

Common mistakes in Efficient Markets and Arbitrage

Saying an efficient market means prices never fall or that no one makes money.
Efficiency is confused with predictability or certainty.
Fix: State that efficiency means no consistent excess return after allowing for risk. Returns still occur and prices still fall.
Mixing up weak and semi-strong form.
Both mention information and the names do not say what is included.
Fix: Link each form to its set: weak is past prices only, semi-strong is all public information, strong is all information.
Saying technical analysis fails only under the strong form.
Students think the strongest form is needed for any failure of analysis.
Fix: Technical analysis already fails if the weak form holds. Weak form efficiency is enough.
Calling any high-return opportunity an arbitrage.
Arbitrage is treated as meaning a good bet.
Fix: Arbitrage needs no net outlay, no risk of loss and a chance of gain. A risky high expected return is not arbitrage.
Forgetting income or costs when finding a forward price.
The simple formula F = S(1 + i)^T is memorised without conditions.
Fix: Adjust for known income (subtract its present value) and state assumptions about transaction costs and equal borrowing and lending rates.
Concluding that markets are fully efficient from one piece of evidence.
Evidence is overstated in written answers.
Fix: Say the evidence supports or contradicts a particular form and mention limits such as transaction costs and insider information.

Worked examples

Example 1

A non-dividend-paying share has a spot price of ₹500. The risk-free rate is 6% per year effective, and borrowing and lending are at this rate with no transaction costs. A one-year forward contract on the share is quoted at ₹540. Show whether an arbitrage exists and find the profit per share at time 1.

Show the solution

The no-arbitrage forward price is F = 500 × 1.06 = ₹530.
The quoted price ₹540 is higher than ₹530, so the forward is overpriced.
Strategy at time 0: sell the forward at ₹540, borrow ₹500 and buy one share. Net cost at time 0 is 0.
At time 1: deliver the share under the forward and receive ₹540.
Repay the loan: 500 × 1.06 = ₹530.
Profit at time 1 = 540 − 530 = ₹10, whatever the share price then.

Answer: An arbitrage exists. Sell the forward, buy the share with borrowed money. The riskless profit is ₹10 per share at time 1.

Example 2

An analyst earns consistently higher returns than the market by buying shares just after quarterly results are published, using only the published figures. State which forms of the efficient market hypothesis this evidence contradicts and which it does not, and explain why.

Show the solution

The analyst uses published results, which are public information.
Public information beyond past prices is the information set of the semi-strong form.
Consistent excess return from public information contradicts the semi-strong form.
The strong form includes all public information, so it is contradicted too.
The weak form concerns only past price and trading data. The strategy does not use these, so the evidence says nothing about the weak form.

Answer: The evidence contradicts the semi-strong and strong forms. It does not contradict the weak form, as the strategy does not use past price data.

Exam tips

Learn the three forms as nested sets of information. Many multiple-choice questions only test this.
In written answers, give a reason with each form, such as what type of analysis would fail.
For arbitrage numbers, state your assumptions first: no transaction costs, no taxes, same borrowing and lending rate.
Show the cash flows at time 0 and at the end, so the marker sees the profit is riskless.
Link efficiency to corporate finance: fair share prices, limited gain from timing issues, and why NPV is the sound basis for decisions.

Practice questions from Key principles of finance

Efficient Markets and Arbitrage 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 and Arbitrage: frequently asked questions

What is the difference between weak and strong form market efficiency?

Weak form says prices reflect all past price and trading data. Strong form says prices reflect all information, public and private. If the strong form holds, the weak form holds too, but not the other way round.

What is arbitrage in finance with an example?

Arbitrage is a riskless profit with no net investment. For example, if a share costs ₹500, the risk-free rate is 6% and a one-year forward is quoted at ₹540, you can sell the forward, borrow ₹500 and buy the share. You lock in a profit of ₹10 at time 1.

Does the efficient market hypothesis mean you cannot make any profit?

No. It means you cannot consistently earn more than the return justified by the risk you take, using the information set in question. Investors still earn normal returns for bearing risk.

Why does no-arbitrage matter for pricing?

If an arbitrage existed, traders would exploit it and move prices until it disappeared. So prices of assets with identical cash flows must be equal. This lets you price forwards, bonds and options by replication.