Cambridge A Level Economics: Efficiency, market failure and externalities (9708) – Revision Notes

Quick recall for sections 7.3 (Efficiency and market failure) and 7.4 (Private costs and benefits, externalities and social costs and benefits) of the Cambridge International AS & A Level Economics (9708) syllabus for examination in 2026, 2027 and 2028. Topic 7 is A Level content, not examined at AS. For full explanations and longer worked examples, read the study guide, then test yourself with the practice questions. See also the course hub, the printable checklist and the free diagnostics. All figures are invented, in fictional markets, with CU meaning currency units.

Definitions to know

Term	Definition
Productive efficiency	Producing at the lowest cost per unit; for a firm, output at minimum AC, where MC = AC; for the economy, on the production possibility curve
Allocative efficiency	Resources go to the goods consumers value most; P = MC (MSB = MSC when externalities exist)
Pareto optimality	No one can be made better off without making someone else worse off
Pareto improvement	At least one person gains and nobody loses
Dynamic efficiency	Efficiency improving over time through investment and innovation
Market failure	The free market leaves resources allocated inefficiently, so welfare could be higher
Externality	A cost or benefit that falls on a third party outside the transaction
Asymmetric information	One party to a transaction knows more than the other
Moral hazard	Behaviour changes because someone else bears the cost of risk

Formulas and relationships

Item	Relationship
Social cost	SC = PC + EC
Marginal social cost	MSC = MPC + MEC
Social benefit	SB = PB + EB
Marginal social benefit	MSB = MPB + MEB
Free market	MPB = MPC
Social optimum	MSB = MSC
Net social benefit	SB − SC
Welfare loss triangle	½ × gap between curves at the market quantity × quantity difference
Total external cost or benefit	Area under MEC or MEB (add up unit by unit if a schedule)

For a firm with TC = a + bQ², AC = a/Q + bQ and MC = 2bQ. Minimum AC is where they meet, at Q = √(a/b).

Productive and allocative efficiency (7.3.1, 7.3.2)

• Productive efficiency is about cost per unit. Allocative efficiency is about what is produced.
• A firm can meet one and miss the other. A profit-maximising monopoly sets MR = MC, so P is above MC.
• Perfect competition in the long run gives P = MC = minimum AC, meeting both conditions.
• Economy-wide, a point inside the production possibility curve is productively inefficient. A point on the curve can still be allocatively inefficient if the mix is not what people want.

Method for a firm.

1. Write AC and MC from TC.
2. Set MC = AC (or find the minimum AC) for productive efficiency.
3. Set P = MC for allocative efficiency.
4. Compare the actual output with each.

Small reminder. For TC = 100 + Q², AC = 100/Q + Q. MC = 2Q. They meet where Q² = 100, so Q = 10 and AC = 20 = MC.

Pareto optimality and dynamic efficiency (7.3.3, 7.3.4)

• Test a move: does anyone lose? If anyone loses, it is not a Pareto improvement.
• Many Pareto optimal points exist, some very unequal. Pareto optimality says nothing about fairness.
• Dynamic efficiency is judged over time. Lower costs and better products from research count, even if prices are above MC now.
• Trade-off: market power can cause static loss but may fund investment. Whether it does depends on how profit is used and on threat of entry.

Market failure and its reasons (7.3.5, 7.3.6)

• Complete failure: no market exists, as with public goods.
• Partial failure: a market exists with the wrong quantity or price.
• Reasons: externalities, public goods, merit and demerit goods (imperfect information), asymmetric information, market power, immobile factors, unequal incomes.
• Evaluate by size. A small externality in a minor market may not justify action, and intervention itself can fail.

Four types of externality (7.4.3, 7.4.4)

	Negative	Positive
Production	MSC above MPC: over-production	MSC below MPC: under-production
Consumption	MSB below MPB: over-consumption	MSB above MPB: under-consumption

Rule: external costs add to cost or subtract from benefit, so they lift MSC or lower MSB. External benefits do the opposite.

Method for a calculation.

1. Write all curves as equations.
2. Market quantity: set MPB = MPC.
3. Social quantity: add MEC or MEB to the correct side, then set MSB = MSC.
4. Find the gap between MSB and MSC at the market quantity.
5. Welfare loss = ½ × gap × difference in quantity.

Worked reminder: a negative externality of consumption

A fictional city centre. Q is in thousand car trips, values in CU.

• MPB = 96 − 4Q
• MPC = 12 + 2Q
• Marginal external cost of consumption = Q, so MSB = 96 − 5Q

1. Market: 96 − 4Q = 12 + 2Q gives 84 = 6Q, so Q = 14 and P = 40.
2. Social optimum: 96 − 5Q = 12 + 2Q gives 84 = 7Q, so Q = 12 and MSB = MSC = 36.
3. Over-consumption = 14 − 12 = 2 thousand trips.
4. At Q = 14, MSB = 96 − 70 = 26 and MC = 12 + 28 = 40, a gap of 14.
5. Welfare loss = ½ × 14 × 2 = 14 thousand CU.

At the social optimum the marginal external cost is 12 CU per trip, which is the size of the gap a corrective tax would aim to close.

Deadweight welfare loss (7.4.5)

• It arises because the market produces a quantity where MSB and MSC differ.
• Negative externality: the loss sits to the right of the social optimum.
• Positive externality: the loss sits to the left of the social optimum.
• On a diagram, shade only the triangle between MSB and MSC. Label the market quantity and the social quantity.

Asymmetric information and moral hazard (7.4.6)

• Asymmetric information: sellers often know more than buyers (used goods, financial products, medical care). Poor decisions follow, and better-quality goods may leave the market.
• Moral hazard: after insurance or a guarantee, the protected party takes more risk.
• Insurer’s example: loss 5,000 CU, chance of a claim 4% before cover and 6% after. Expected claim rises from 200 to 300 CU. A premium of 200 loses 100 CU per policy.
• Responses: excesses, monitoring, sharing information, regulation. Each has a cost.

Costs and benefits in decisions (7.4.7)

Method for a cost-benefit decision.

1. List private and external costs, and private and external benefits.
2. Give each a money value.
3. Add up SB and SC.
4. Net social benefit = SB − SC. Positive means go ahead.
5. Test sensitivity: which values are least certain?

Net present value is not required. A decision with positive net social benefit but large uncertain values deserves caution.

Quick self-test

1. State the condition for productive efficiency for a firm.
2. State the condition for allocative efficiency when externalities exist.
3. At the 40th unit, MPC = 22 and MEC = 7. Find MSC.
4. PB = 340 and EB = 90. Find SB.
5. A change gives person A two more units of utility and leaves B unchanged. Is it a Pareto improvement?
6. For TC = 100 + Q², find the output and AC at which the firm is productively efficient.
7. MPB = 80 − 4Q, MPC = 20 + 2Q, and MEC is a constant 6. Find the market quantity, the social optimum and the welfare loss.
8. Give an example of a positive externality of production.
9. Why is moral hazard a problem for insurers?
10. SB = 48 and SC = 51 (CU million). Should the project go ahead on these figures?
11. Why can a monopoly be dynamically efficient but statically inefficient?

Answers

1. Output at the minimum of AC, where MC = AC.
2. MSB = MSC.
3. 22 + 7 = 29.
4. 340 + 90 = 430.
5. Yes. A gains and nobody loses.
6. AC = 100/Q + Q and MC = 2Q. Q² = 100, so Q = 10 and AC = 20.
7. Market: 80 − 4Q = 20 + 2Q gives Q = 10, P = 40. Social: MSC = 26 + 2Q, so 54 = 6Q and Q = 9, with MSB = MSC = 44. At Q = 10, MSC = 46 and MPB = 40, a gap of 6, so welfare loss = ½ × 6 × 1 = 3.
8. A firm that trains workers who later move to other firms; or a firm whose research is used by others.
9. Cover makes the insured take more risk, so claims exceed the level the premium assumed.
10. No. Net social benefit = 48 − 51 = −3, but check how uncertain the values are.
11. It may price above MC (static loss) but use the profit to cut costs or create products over time.

Where marks are usually lost

• Writing “P = MC” for productive efficiency, or “MC = AC” for allocative.
• Stating a diagram result without naming the curves, the market quantity or the social optimum.
• Putting MEC on the benefit curve or MEB on the cost curve.
• Shading a large area as welfare loss when only the triangle between MSB and MSC counts.
• Using the wrong direction: a negative externality gives over-production, a positive one under-production.
• Calling any gain a Pareto improvement even though someone loses.
• Mixing up asymmetric information (before the deal) with moral hazard (after the deal).
• Concluding a cost-benefit result without commenting on uncertainty, intangibles or who is affected.
• Ending a calculation without a unit or a plain-English conclusion.

Official syllabus

Cambridge International, Cambridge International AS & A Level Economics (9708) syllabus for examination in 2026, 2027 and 2028, Subject content, topic 7, sections 7.3 and 7.4.