📚 IB CIE Economics: Common Mistakes Explained | IB CIE 经济：易错题精讲
IB and CIE Economics exams reward precise thinking. Yet every year, capable students drop marks by repeating a handful of predictable errors — misreading diagrams, swapping supply and demand shifts, or muddling elasticity signs. This article pinpoints those high-cost mistakes and explains how to get them right, with paired English–Chinese explanations for clarity.
IB 和 CIE 经济学考试奖励精准的思维，但每年都有不少能力不错的学生因为反复出现的一些典型错误而丢分——误读图表、混淆供给与需求的移动、把弹性符号弄反等等。本文精准锁定这些高代价的易错点，并给出中英双语对照讲解，帮你彻底理清。
1. Movement vs. Shift of Demand Curve | 需求量的变动与需求变动的混淆
A classic trap: ‘An increase in the price of coffee reduces demand for coffee.’ Wrong. A price change causes a movement along the demand curve, not a shift. The correct statement is: it reduces the quantity demanded of coffee. A shift of the entire demand curve only happens when a non-price determinant changes, such as income, tastes, or the price of related goods.
一个经典陷阱：“咖啡涨价导致咖啡需求下降。”错。价格变化引起的是沿着需求曲线的移动，而不是需求曲线的平移。正确说法是：咖啡的需求量下降。整条需求曲线的平移只会在非价格决定因素发生变化时出现，比如收入、偏好或相关商品的价格。
Now apply this: if the price of butter falls, the demand for margarine (a substitute) will shift left. Students often mistakenly say ‘the quantity demanded of margarine decreases’, forgetting that it is a shift in demand caused by a change in a related good’s price.
再应用一下：如果黄油价格下降，人造黄油（替代品）的需求曲线将向左平移。学生常误说成“人造黄油的需求量减少”，忘记这是由相关商品价格变动引起的需求平移。
- Key tip: Price of this good → movement along curve; other factors → shift of curve.
- 关键词：自身价格→线上移动；其他因素→整条线平移。
2. Price Elasticity of Demand Calculation | 需求价格弹性的计算陷阱
When calculating PED, many candidates simply use the starting values: (Q2 − Q1)/Q1 ÷ (P2 − P1)/P1. This gives a different result depending on the direction of change. CIE and IB examiners expect the midpoint formula for elasticity between two points on a demand curve, unless the question specifies a point method.
在计算需求价格弹性时，很多考生直接用初始值来算：(Q2 − Q1)/Q1 ÷ (P2 − P1)/P1。这样做随着价格变化方向的不同会得到不同的结果。CIE 和 IB 考官要求使用中点公式来计算需求曲线上两点之间的弹性，除非题目明确指定点弹性法。
PED = (ΔQ ÷ Average Q) ÷ (ΔP ÷ Average P) = [(Q2−Q1) / ((Q1+Q2)/2)] ÷ [(P2−P1) / ((P1+P2)/2)]
A second common error is ignoring the negative sign when interpreting PED. PED is almost always negative for normal goods, but students often state ‘PED = 2’ and then claim demand is elastic. While the absolute value is used for classification, forgetting the negative sign can cause confusion in essay chains about revenue.
第二个常见错误是在解释 PED 时忽略负号。对正常商品而言 PED 几乎总是负数，但学生常常只说“PED = 2”然后声称需求富有弹性。虽然分类时用绝对值，但忘掉负号会在论述收益变化的链条中造成混乱。
3. Cross Elasticity and Sign Errors | 交叉弹性符号的常见错误
The sign of cross elasticity of demand (XED) is critical: positive for substitutes, negative for complements. A frequent mistake is writing a positive XED for petrol and cars, or a negative XED for tea and coffee. Always think about the relationship first — if the price of one rises, do consumers buy more or less of the other?
需求交叉弹性 (XED) 的符号至关重要：替代品为正，互补品为负。一个常见错误是把汽油和汽车写成正 XED，或者把茶和咖啡写成负 XED。务必先想清楚两种商品的关系——如果一种商品的价格上涨，消费者对另一种的购买量是增加还是减少了？
Similarly, income elasticity of demand (YED) reveals whether a good is normal (positive YED) or inferior (negative YED). A miscalculated sign can flip the entire analysis in a data-response question. For luxury goods, YED > 1; for necessities, 0 < YED < 1. Inferior goods have negative YED, not 'elastic YED'.
同样，需求收入弹性 (YED) 反映商品是正常品（YED 为正）还是低档品（YED 为负）。一旦符号算错，整个数据分析题的分析都会反转。奢侈品的 YED > 1；必需品的 YED 在 0 到 1 之间；低档品的 YED 为负，而不是“富有弹性”。
4. Consumer and Producer Surplus in Diagrams | 消费者剩余与生产者剩余的图示误判
After a tax or subsidy, students often shade the wrong triangles. Consumer surplus is always the area below the demand curve and above the price consumers actually pay. Producer surplus is the area above the supply curve and below the price producers receive. With an indirect tax, the consumer price rises to Pc, the producer price falls to Pp, and the government rectangle is the tax per unit times the new quantity.
在征税或补贴之后，学生常常会涂错三角形区域。消费者剩余永远是需求曲线以下、消费者实际支付价格以上的面积。生产者剩余则是供给曲线以上、生产者实际得到的价格以下的面积。征收间接税时，消费者价格升至 Pc，生产者价格降至 Pp，政府所得矩形则是单位税额乘以新数量。
A typical error: in a tariff diagram, confusing the change in consumer surplus with the deadweight loss. The tariff raises domestic price, so consumer surplus falls by a large trapezium; part becomes producer surplus, part government revenue, and two small triangles are deadweight loss. Double-check each area before labelling.
一个典型错误：在关税图中混淆消费者剩余的变化与无谓损失。关税提高了国内价格，因此消费者剩余减少了一大块梯形；其中一部分转化为生产者剩余，一部分成为政府收入，两个小三角形则是无谓损失。标注之前一定要反复检查每一块面积。
5. Negative Externalities and Social Optimum | 负外部性与社会最优的图示错误
When drawing negative production externalities, students often swap the Marginal Private Cost (MPC) and Marginal Social Cost (MSC) curves. The MSC curve should lie to the left (or above) the MPC curve because social cost includes the external cost. The free market produces where MPB = MPC, leading to overproduction compared with the social optimum where MSB = MSC.
在画负生产外部性时，学生经常把边际私人成本 (MPC) 和边际社会成本 (MSC) 曲线位置画反。MSC 曲线应位于 MPC 曲线的左侧（或上方），因为社会成本包含了外部成本。自由市场在 MPB = MPC 处生产，相比 MSB = MSC 的社会最优产量，造成了过度生产。
Another blunder: mislabelling the welfare loss triangle. The deadweight loss is the area between MSC and MSB curves from the private optimum quantity to the social optimum quantity — not the whole area between the two cost curves. Always shade the triangle that represents the net social loss.
另一个失误：错标福利损失三角形。无谓损失是 MSC 和 MSB 曲线之间、从私人最优产量到社会最优产量所夹的三角形区域，而不是两条成本曲线之间的全部面积。一定要涂出代表社会净损失的那个三角形。
6. AD-AS Model and Long-Run Equilibrium | AD-AS 模型与长期均衡的误解
Students frequently treat the short-run aggregate supply (SRAS) curve as vertical, or the long-run aggregate supply (LRAS) as upward sloping. In the classical model, LRAS is vertical at the full-employment level of output, because in the long run wages and prices are fully flexible. A recessionary gap means equilibrium output is below full employment, shown by an intersection of AD and SRAS to the left of LRAS.
学生经常把短期总供给 (SRAS) 画成垂线，或把长期总供给 (LRAS) 画成向上倾斜。在古典模型中，LRAS 在充分就业产出水平上是一条垂线，因为在长期中工资和价格可以充分调整。衰退缺口意味着均衡产出低于充分就业水平，表现为 AD 与 SRAS 的交点位于 LRAS 的左侧。
When explaining self-adjustment, many candidates do not describe the mechanism clearly. Deflationary gap → falling wages → SRAS shifts right, returning output to Yf. But they often draw AD shifting right instead, confusing fiscal stimulus with automatic adjustment. Make the distinction explicit.
在解释自动调整机制时，很多考生的描述并不清晰。通缩缺口 → 工资下降 → SRAS 右移，使产出回到 Yf。但他们常常画成 AD 右移，把财政刺激与自动调整混为一谈。务必将两者明确区分开来。
7. The Multiplier Effect and Leakages | 乘数效应与漏出的疏忽
A simple formula: multiplier k = 1/(1−MPC) or 1/MPS. But students often ignore the fact that in an open economy with government, leakages include savings (S), taxes (T) and imports (M). The full multiplier becomes k = 1 / [MPS + MPT + MPM], where MPT is the marginal propensity to tax and MPM is the marginal propensity to import. Using the simple MPC formula without considering these will overstate the multiplier.
简单公式：乘数 k = 1/(1−MPC) 或 1/MPS。但学生往往忽略了一点：在有政府的开放经济中，漏出项包括储蓄 (S)、税收 (T) 和进口 (M)。完整的乘数变为 k = 1 / [MPS + MPT + MPM]，其中 MPT 为边际税收倾向，MPM 为边际进口倾向。如果不考虑这些漏出，直接套用简单的 MPC 公式，就会高估乘数的大小。
Another frequent pitfall: computing the change in national income as ‘initial injection × multiplier’, but forgetting that the injection may have been partly offset by a simultaneous tax increase. Always read the question carefully for any policy combination.
另一个常见陷阱：计算国民收入变化时用了“初始注入 × 乘数”，但忘了某笔注入可能同时被增税部分抵消。一定要仔细审题，留意任何政策组合。
8. Inflation Types and the Phillips Curve | 通货膨胀类型与菲利普斯曲线混淆
Demand-pull inflation is caused by an increase in aggregate demand, shifting AD right and raising both price level and real output in the short run. Cost-push inflation is caused by a supply shock, shifting SRAS left, raising prices but lowering output. Students often depict cost-push inflation by shifting AD right — a mistake that suggests expansionary policy is to blame, ignoring the supply side.
需求拉动型通胀是由总需求增加引起的，AD 右移，短期中价格水平和实际产出双双上升。成本推动型通胀则是由供给冲击引发的，SRAS 左移，价格上升但产出下降。学生常常把成本推动型通胀画成 AD 右移——这个错误会让人误以为是扩张性政策造成的，完全忽略了供给端。
Regarding the Phillips Curve, remember the short-run curve slopes downward, showing a trade-off between inflation and unemployment. The long-run Phillips Curve is vertical at the natural rate of unemployment, because any attempt to push unemployment below the natural rate only accelerates inflation. Mixing up the two curves loses easy marks on policy evaluation.
关于菲利普斯曲线，记住短期曲线向下倾斜，表示通胀与失业之间的取舍关系。长期菲利普斯曲线在自然失业率处垂直，因为任何想把失业率压低到自然率以下的尝试，都只会导致通胀加速。把两种曲线搞混会在政策评价题中轻易丢分。
9. Comparative Advantage and Terms of Trade | 比较优势与贸易条件的计算错误
Comparative advantage is determined by lower opportunity cost, not by absolute output numbers. When given a production table, always calculate the opportunity cost of producing one unit of each good in each country. A classic error is to compare output per hour and assume the country with higher absolute output in both goods gains nothing from trade — comparative advantage principles still hold.
比较优势由较低的机会成本决定，而不是看绝对产量数字。在给出生产成本或产量表时，一定要先把每个国家生产每种商品的机会成本算出来。一个经典错误是比较每小时产出后，认为在两个商品上都拥有绝对优势的国家无法从贸易中获益——但比较优势原理依然成立。
| Country | 1 unit of Cloth costs | 1 unit of Wheat costs |
|---|---|---|
| A | 2/3 Wheat | 1.5 Cloth |
| B | 1 Wheat | 1 Cloth |
If Country A exports cloth, Country B exports wheat, the terms of trade must lie between the two opportunity cost ratios: 2/3 wheat per cloth and 1 wheat per cloth. Proposing a ratio outside this range means one country would be better off producing the good domestically, a common fault in 6‑mark trade questions.
如果 A 国出口布，B 国出口小麦，贸易条件必须落在两个机会成本比率之间，即每单位布换 2/3 到 1 单位小麦之间。如果提出一个超出此范围的交换比例，就会使其中一国选择自己生产而非贸易，这是贸易部分 6 分题中常见的问题。
10. Exchange Rates and the J‑Curve Effect | 汇率与 J 曲线效应的分析误区
When evaluating a currency depreciation, students often immediately assume the current account improves. In reality, the J‑curve effect means the trade balance may worsen in the short run before it gets better. Import contracts fixed in foreign currency, coupled with price-inelastic short-run demand for imports and exports, cause import spending to rise and export revenue to fall initially.
在评价本币贬值时，学生往往立刻假设经常账户会改善。实际上，J 曲线效应意味着短期内贸易收支反而可能恶化，然后才会好转。以外币计价的进口合同，加上进出口的短期需求缺乏弹性，会导致进口支出先上升、出口收入先下降。
Only over time, as quantities adjust, does the depreciation start to boost net exports — provided the Marshall‑Lerner condition holds (sum of export and import price elasticities > 1). A common mistake is to label a simple leftward shift of the exchange rate as an automatic cure for all trade deficits, ignoring the time lag and elasticity conditions.
只有随着时间的推移、数量开始调整以后，贬值才会开始刺激净出口——前提是马歇尔‑勒纳条件成立（进出口价格弹性之和大于 1）。常见错误是把汇率左移直接标为贸易逆差的万能药，完全忽视了时滞和弹性条件。
Published by TutorHao | Economics Revision Series | aleveler.com
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