A-Level Economics: Commonly Mistaken Questions Explained | A-Level 经济：易错题精讲

📚 A-Level Economics: Commonly Mistaken Questions Explained | A-Level 经济：易错题精讲

Many A-Level Economics students struggle not with grasping theories but with applying them accurately to exam questions. Common mistakes range from mixing up demand and quantity demanded to misinterpreting elasticity calculations. This article presents a series of typical exam-style errors, explains the underlying concepts, and shows how to avoid losing marks.

许多A-Level经济学学生不是不理解理论，而是在考试题目中准确应用时遇到困难。常见错误包括混淆需求与需求量、误解弹性计算等。本文呈现一系列典型考题错误，解释背后概念，并展示如何避免丢分。

1. Movement Along vs. Shift of Demand Curve | 需求曲线移动与沿需求曲线的变动

A classic misconception: ‘An increase in price reduces demand, so the demand curve shifts left.’ Actually, a price change causes a movement along the demand curve (contraction in quantity demanded). A shift of the demand curve occurs only when a non-price determinant (income, tastes, price of substitutes, etc.) changes. In exam questions, if the prompt says ‘due to a rise in price,’ the correct answer is a movement up the demand curve, not a shift.

经典误区：’价格上升导致需求减少，因此需求曲线向左移动。’事实上，价格变动引起的是沿需求曲线的移动（需求量收缩）。需求曲线的移动仅发生在非价格决定因素（收入、偏好、替代品价格等）变化时。在考试中，若题目说’由于价格上涨’，正确答案是沿需求曲线向上移动，而非曲线本身的移动。

A typical incorrect multiple-choice: ‘If the price of coffee increases, the demand for coffee…’ many answer ‘shifts left.’ The correct answer: quantity demanded decreases, movement along the curve. Similarly, a decrease in the price of a complementary good (e.g., milk) shifts the demand curve for coffee rightward. Always distinguish price factor vs. non-price factor.

典型错误选择题：’如果咖啡价格上涨，咖啡的需求……’许多人选’向左移动’。正确答案：需求量减少，沿需求曲线移动。类似地，互补品（如牛奶）价格下降会使咖啡的需求曲线右移。务必区分价格因素与非价格因素。

2. Price Elasticity of Demand and Total Revenue | 需求价格弹性与总收益

Students often confuse the relationship between elasticity and total revenue. If demand is price elastic (PED > 1), a price reduction increases total revenue because the percentage increase in quantity demanded exceeds the percentage fall in price. Conversely, if demand is price inelastic (PED < 1), raising price raises total revenue. A common error is assuming that a price hike always boosts revenue or that any discount increases sales enough to raise revenue.

学生们常混淆弹性与总收益的关系。若需求富有价格弹性（PED > 1），降价会使总收益增加，因为需求量的增加百分比超过价格下降百分比。相反，若缺乏弹性（PED < 1），提价会使总收益增加。常见错误是假设涨价总能增收，或任何折扣都能使销量增加到足以提高收益。

Example mistake: A firm raises price by 10% and PED = 1.5. Some calculate new total revenue incorrectly. Since PED > 1, the percentage decrease in quantity demanded is 15% (1.5 × 10%). Revenue = new price × new quantity; it falls. Yet many students get the direction wrong. Use the approximation: %ΔTR ≈ %ΔP + %ΔQ. With %ΔP = +10% and %ΔQ = -15%, TR changes by about -5%.

错误示例：某公司提价10%，PED = 1.5。有人计算新总收益时出错。由于PED>1，需求量减少百分比为15% (1.5×10%)。收益=新价格×新数量，收益下降。但很多学生搞错方向。可用公式估算：%ΔTR ≈ %ΔP + %ΔQ。%ΔP=+10%，%ΔQ=-15%，总收益约降5%。

3. Consumer Surplus and Producer Surplus in Diagrams | 消费者剩余与生产者剩余在图中的计算

A frequent error is miscalculating the areas of consumer and producer surplus after a price change or tax. Students may include the wrong triangle or forget to adjust for new equilibrium. In a supply-demand diagram, consumer surplus is the area below the demand curve and above the price, up to quantity traded. A tax shifts the supply curve left, creating a deadweight loss. The new consumer surplus is the triangle up to the new price, not the original.

常见错误是价格变动或征税后消费者剩余和生产者剩余的面积计算错误。学生可能选错三角形或忘记根据新均衡调整。在供需图中，消费者剩余是需求曲线下方、价格上方直至交易量的区域。征税使供给曲线左移，产生无谓损失。新的消费者剩余是直到新价格的三角形，而非原来的。

Example: Originally equilibrium price £10, quantity 100. A specific tax of £3 shifts supply; new price £12, quantity 80. Consumer surplus shrinks from a larger triangle to a smaller one. Many answer incorrectly by using the original quantity or misidentifying the height. Remember: consumer surplus = ½ × base (quantity) × height (max willingness to pay minus actual price). Double-check the new post-tax price and quantity.

示例：原均衡价格10镑，数量100。征从量税3镑，供给移动；新价格12镑，数量80。消费者剩余从较大的三角形缩小为较小的。很多人用原数量或错误识别高度而答错。记住：消费者剩余=½ × 底（数量）× 高（最大支付意愿减去实际价格）。重新检查税后新价格和数量。

4. Fixed, Variable, and Sunk Costs | 固定成本、可变成本与沉没成本

In the short run, fixed costs do not vary with output, but students sometimes mistakenly classify rent as a variable cost or raw materials as fixed. Another trap: sunk costs. A sunk cost is an expenditure that cannot be recovered. When making forward-looking decisions, firms should ignore sunk costs. However, many students include sunk costs when calculating shutdown points or continuation decisions.

在短期，固定成本不随产量变化，但学生有时误将租金归为可变成本，或将原材料归为固定成本。另一个陷阱：沉没成本。沉没成本是已经发生且无法收回的支出。在做未来决策时，企业应忽略沉没成本。但许多学生在计算停业点或继续经营决策时却计入沉没成本。

A classic question: ‘A firm paid £50,000 for a machine last year. It could sell it for £20,000 now. Should it consider the original price when deciding whether to produce?’ The £50,000 is sunk; only the opportunity cost (the £20,000 resale value) matters. Misunderstanding this leads to the fallacy of ‘throwing good money after bad.’

经典考题：’一家公司去年支付了5万英镑购买一台机器，现在可卖2万英镑。在决定是否生产时，它应考虑原始价格吗？’5万是沉没成本；只有机会成本（2万的转售价值）才相关。误解这一点会导致’沉没成本谬误’。

5. Perfect vs. Monopolistic Competition | 完全竞争与垄断竞争的区别

Students frequently confuse these two market structures. Both have many firms and low barriers to entry, but key differences: in perfect competition, products are homogeneous, firms are price takers, and there is perfect information. Monopolistic competition features product differentiation, giving firms some price-making power, and advertising is common. A common mistake is thinking that monopolistic competitors earn supernormal profits in the long run; they do not due to free entry, they earn normal profit just like perfect competition (but with excess capacity).

学生常混淆这两种市场结构。两者都有许多企业、低进入壁垒，但关键区别：完全竞争中产品同质，企业是价格接受者，信息完全。垄断竞争则存在产品差异化，企业有部分定价权，且广告常见。常见错误是认为垄断竞争企业长期能获超额利润；实际上由于自由进入，它们也只能获得正常利润，与完全竞争类似（但存在过剩产能）。

An exam diagram question may show a downward-sloping demand curve for a firm, indicating monopolistic competition, whereas a horizontal demand curve indicates perfect competition. Mislabeling leads to lost marks. Also remember: in monopolistic competition, price > marginal cost in the short run, but entry drives demand left until tangent to ATC in the long run.

考题中图示题可能展示企业的向下倾斜需求曲线，表明垄断竞争，而水平需求曲线则指示完全竞争。标记错误导致丢分。还要记住：垄断竞争短期价格>边际成本，但进入使需求左移，直到长期需求曲线与平均总成本线相切。

6. Positive and Normative Statements | 实证表述与规范表述

One of the easiest topics to slip up on: positive statements are objective and testable (‘Unemployment is 6%’), normative statements involve value judgments (‘Unemployment is too high’). However, examiners often use tricky wording like ‘should’ or ‘ought to’ to indicate normative. A sentence containing ‘it is argued that…’ may still be positive if it reports a claim factually. A statement like ‘The government should increase spending’ is normative, while ‘Increased spending will raise aggregate demand’ is positive.

最容易失分的基本概念之一：实证表述是客观可检验的（’失业率为6%’），规范表述涉及价值判断（’失业率太高了’）。但考官常使用’应该”必须’等词来表明规范性。包含’有人认为……’的句子，如果是客观报道一个主张，仍可能为实证。’政府应该增加支出’是规范表述，而’增加支出将提高总需求’则是实证表述。

A common wrong answer: The statement ‘Income inequality has widened in the past decade’ is positive; ‘The government must act to reduce inequality’ is normative. In multiple-choice, if you see ‘should’ or ‘fair’ or ‘desirable’, it’s almost always normative. Read carefully: sometimes a statistic is sandwiched between normative phrases, but the statistic itself remains positive.

常见错误选项：’过去十年收入不平等扩大了’是实证；’政府必须采取行动减少不平等’是规范。选择题中看到’应该”公平”可取’等词，几乎都是规范表述。要仔细：有时数据夹在规范性短语中，但数据本身仍属实证。

Published by TutorHao | A-Level Economics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）