📚 A-Level WJEC Economics: Common Pitfalls & Model Answers | A-Level WJEC 经济：易错题精讲
This article dives into some of the most common mistakes made by A-Level WJEC Economics students and provides clear, model explanations to help you avoid them. Understanding tricky concepts like elasticity calculations, the multiplier in an open economy, and the distinction between different types of unemployment can substantially boost your exam performance.
本文深入剖析 A-Level WJEC 经济学科中学生最常犯的错误，并给出清晰的范例讲解，帮助你避开这些陷阱。理解需求弹性计算、开放经济乘数以及各类失业的区别等微妙概念，能大幅提升你的考试成绩。
1. Misinterpreting Price Elasticity of Demand and Revenue | 误解需求价格弹性与收益
A typical pitfall is confusing how a price change affects total revenue depending on PED. Many students mistakenly think that when demand is elastic, raising the price always increases revenue. The correct rule: if PED > 1 (elastic), a price rise reduces total revenue; if PED < 1 (inelastic), a price rise increases total revenue.
一个典型陷阱是混淆在不同需求价格弹性下，价格变动对总收入的影响。许多学生误以为富有弹性时提价总能增加收入。正确规则是：若 PED > 1（富有弹性），涨价会使总收入减少；若 PED < 1（缺乏弹性），涨价会使总收入增加。
Sample exam error: “A firm’s PED = –2. It increases price by 10%. State the change in quantity demanded and the effect on total revenue.” The common wrong answer claims quantity falls by 10% and revenue is unchanged. In reality, %ΔQd = PED × %ΔP = –2 × 10% = –20%. New TR = 1.1P × 0.8Q = 0.88 of original revenue, a 12% decrease. Revenue therefore falls, not stays flat.
考题常见错误：”某企业 PED = –2，提价 10%。求需求量变化的百分比及总收入如何变化。” 常见错误答案是需求量下降 10%，收入不变。实际上，需求量的变化百分比为 PED × 价格变动% = –2 × 10% = –20%。新总收入为 1.1P × 0.8Q，降至原来的 88%，即下降12%。因此收入减少，而非不变。
Key reminder: always apply the percentage change correctly and test total revenue with the new price and quantity.
要点提示：务必正确计算百分比变动，并用新价格和新数量检验总收入。
2. The Open-Economy Multiplier | 开放经济乘数
Students frequently use the simple multiplier k = 1/(1–MPC) even when the question provides a proportional tax rate and marginal propensity to import (MPM). This ignores significant leakages and overstates the multiplier.
学生经常在题目给出了比例税率和边际进口倾向时，仍然使用简单乘数 k = 1/(1–MPC)。这忽略了重要的漏出项，夸大了乘数效应。
Typical mistake: given MPC = 0.8, tax rate t = 0.25, MPM = 0.1, a student calculates k = 1/(1–0.8) = 5. The correct formula in an open economy with a proportional tax is:
典型错误：已知 MPC = 0.8，税率 t = 0.25，MPM = 0.1，学生算出 k = 1/(1–0.8) = 5。在含比例税的开放经济中，正确公式为：
k = 1 / (1 – MPC(1 – t) + MPM)
Substituting: k = 1 / (1 – 0.8 × 0.75 + 0.1) = 1 / (1 – 0.6 + 0.1) = 1 / 0.5 = 2. Thus a £100m injection raises national income by only £200m, not £500m.
代入计算：k = 1 / (1 – 0.8 × 0.75 + 0.1) = 1 / (1 – 0.6 + 0.1) = 1 / 0.5 = 2。因此，1亿英镑的注入只能带来2亿英镑的国民收入增加，而非5亿。
3. Perfect Competition: Profit Maximisation Rule | 完全竞争：利润最大化规则
In perfect competition, the profit-maximising output occurs where price = marginal cost (MC), not where total revenue equals total cost. A common error is to set MR = 0 or to confuse the shutdown point with the optimum.
在完全竞争市场中，利润最大化产量在价格等于边际成本处，而非总收入等于总成本处。常见错误是设边际收益为零，或把停业点与最优点混淆。
Example: Market price = £10, TC = 5 + Q². The wrong approach tries to find where TR = TC, giving Q ± 5, but this yields zero profit. The correct method: P = MR = 10, MC = 2Q. Set 10 = 2Q, so Q = 5 units. Profit = TR – TC = 50 – (5+25) = £20.
案例：市场价格 = £10，总成本 TC = 5 + Q²。错误做法是令 TR = TC，解出 Q 约 5，但利润为零。正确方法：在完全竞争下，P = MR = 10，MC = 2Q。令 10 = 2Q，得 Q = 5 单位。利润 = TR – TC = 50 – (5+25) = £20。
Always remember: the firm maximises profit where MR = MC; in perfect competition MR = price. Whether it makes a profit or loss depends on the comparison between price and average total cost at that output.
切记：企业利润最大化点在 MR=MC；完全竞争中 MR=价格。盈利与否取决于该产量下价格与平均总成本的比较。
4. Terms of Trade Calculation | 贸易条件计算
The terms of trade (TOT) index measures a country’s export prices relative to its import prices. It is calculated as:
贸易条件指数衡量一国出口价格相对于进口价格的水平。计算公式为：
TOT = (Export Price Index / Import Price Index) × 100
A frequent mistake is reversing the numerator and denominator. Another error is assuming that any number above 100 means an improvement, when the starting base must be considered.
常见错误是分子分母颠倒，或者认为只要数值大于100就是改善，而忽略了基准年的比较。
Example: Export price index rises from 100 to 120, import price index rises from 100 to 150. A student might calculate 150/120 × 100 = 125 and claim an improvement. The correct TOT = (120/150) × 100 = 80, a deterioration from the base year 100. The country can now buy fewer imports with its exports.
举例：出口价格指数从 100 升至 120，进口价格指数从 100 升至 150。学生可能误算为 150/120 × 100 = 125，并声称改善。正确的 TOT = (120/150) × 100 = 80，与基年 100 相比是恶化，说明该国用等量出口换取的进口减少。
5. The Marshall-Lerner Condition and Exchange Rates | 马歇尔-勒纳条件与汇率
Many students believe that a currency depreciation will automatically improve the trade balance. This is a classic trap. The Marshall-Lerner condition states that depreciation improves the current account only if the sum of the price elasticities of demand for exports and imports (in absolute values) exceeds one.
很多学生以为货币贬值会自动改善贸易收支。这是一个经典陷阱。马歇尔-勒纳条件指出，只有当出口需求的价格弹性与进口需求的价格弹性绝对值之和大于 1 时，贬值才能改善经常账户。
Consider a country that depreciates its currency by 15%. Export demand elasticity = 0.5, import demand elasticity = 0.4. The sum is 0.9 (<1). In the short run, the trade deficit may widen, illustrating the J-curve effect, because import prices rise immediately while volumes adjust slowly.
考虑一国货币贬值 15%，出口需求弹性为 0.5，进口需求弹性为 0.4。弹性之和为 0.9（<1）。短期内贸易赤字可能恶化，呈现 J 曲线效应，因为进口价格立刻上升，而数量调整缓慢。
Exam tip: always check the sum of elasticities before concluding the effect of exchange rate changes. Write “Marshall-Lerner condition” explicitly to earn marks.
答题提示：在判断汇率变动的影响前，务必检查弹性之和。明确写出 “马歇尔-勒纳条件” 以获取分数。
6. Cyclical vs. Structural Unemployment | 周期性失业与结构性失业
Candidates often misclassify unemployment caused by a recession as “structural”. Cyclical unemployment (demand-deficient) arises from a fall in aggregate demand, while structural unemployment is due to a mismatch of skills or geographic immobility.
考生经常把经济衰退造成的失业归为 “结构性”。周期性失业（需求不足失业）源于总需求下降，而结构性失业是由技能错配或地理固定性造成的。
Example: A textile factory closes because national consumer spending falls during a downturn, laying off 200 workers. A student labels this structural unemployment, thinking the industry is in decline. In fact, it is cyclical: if aggregate demand recovers, spending and jobs could return. Structural unemployment occurs when workers’ skills do not match new vacancies, e.g., replaced by automation.
案例：一家纺织厂因经济低迷导致全国消费支出下降而关闭，裁员 200 人。学生将其标为结构性失业，以为行业衰落。实际上这属于周期性失业：若总需求恢复，支出和就业可能回归。结构性失业发生在工人技能与新岗位不匹配时，例如被自动化取代。
To distinguish, ask: “Is the primary cause a lack of demand, or a fundamental change in the economy’s structure?”
区分时可问：”根本原因是需求不足，还是经济结构的根本性变化？”
7. Investment and the Production Possibility Frontier | 投资与生产可能性边界
A common mistake is to say that an increase in capital investment immediately shifts the PPF outward. In the short run, producing more capital goods means sacrificing current consumption goods – a movement along the curve, not a shift.
常见错误是认为资本投资增加会立即让生产可能性边界外移。短期内，生产更多资本品意味着牺牲当前的消费品——这是沿着曲线移动，而非曲线的位移。
Example: An economy shifts resources from making consumer electronics to manufacturing industrial robots. The immediate effect is a reallocation along the existing PPF: fewer smartphones, more robots. The outward shift of the PPF occurs only in the long run when the increased capital stock enhances the economy’s productive capacity.
例子：一个经济体将资源从生产消费电子产品转向制造工业机器人。立竿见影的效果是沿现有 PPF 重新分配：手机减少，机器人增加。PPF 的外移只在长期当增加的资本存量提高了经济的生产能力时才会发生。
Thus, “investment causes the PPF to shift out” must be accompanied by “in the long run”. Many students lose marks by omitting the time distinction.
因此，”投资导致 PPF 外移” 的说法必须加上 “在长期”。很多学生因忽略时间区分而丢分。
8. Crowding-Out Effect of Expansionary Fiscal Policy | 扩张性财政政策的挤出效应
A typical error is to assume that an increase in government spending will boost aggregate demand by the full multiplier effect, forgetting that it can push up interest rates and crowd out private investment. This weakens the final impact on GDP.
一个典型错误是，认为政府支出增加能通过完整的乘数效应提高总需求，而忘记了它可能推高利率、挤出私人投资，从而削弱对 GDP 的最终影响。
Suppose the government raises spending by £20bn, and the simple multiplier is 2. A student predicts a £40bn rise in AD. However, due to higher interest rates, private investment falls by £5bn. The net increase in autonomous spending is £15bn, so AD rises by 2 × £15bn = £30bn, not £40bn.
假设政府增支£200亿，简单乘数为2。学生预测 AD 上升£400亿。然而，由于利率上升，私人投资减少£50亿。自主支出的净增量为£150亿，因此 AD 上升 2 × £150亿 = £300亿，而非£400亿。
Remember: partial crowding out
Published by TutorHao | A-Level Economics Revision Series | aleveler.com
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