📚 Year 12 Cambridge Economics: Case Study Practical Drill | Year 12 剑桥经济：案例分析实战演练
Case study questions in Cambridge AS Economics require you to apply theories to real‑world situations. This drill walks you through the analysis of England’s 5p charge on single‑use plastic bags, a classic example of a Pigouvian tax addressing negative externalities. By the end, you will have a clear framework for tackling any case study on government intervention and market failure.
剑桥AS经济考试中的案例分析题要求你把理论运用到实际情境中。本演练将以英格兰对一次性塑料袋征收5便士费用为例，带你逐步分析这个通过庇古税应对外部性的经典案例。读完本文，你将掌握一套清晰的框架，可以用来作答任何关于政府干预与市场失灵的分析题。
1. Understanding the Case Background | 理解案例背景
Begin by reading the provided extract actively. Underline quantitative data such as the 5p charge, the drop in bag usage from 140 to 1.8 bags per person per year, and the £29 million raised for good causes in the first year.
主动阅读提供的案例材料。划出定量数据，比如5便士的收费额、人均塑料袋使用量从每年140个降至1.8个，以及第一年即为公益事业筹集的2900万英镑。
Identify the key stakeholders: consumers (shoppers), retailers (large shops with over 250 employees), the government, the environment, and charity organisations. Also note the timeline—England introduced the charge in 2015, with earlier successes in Wales and Scotland.
识别关键利益相关者：消费者（购物者）、零售商（拥有250名以上员工的大型商店）、政府、环境以及慈善机构。同时留意时间线——英格兰于2015年引入该收费，此前威尔士和苏格兰已取得成功。
Contextualise the issue: single‑use plastic bags are lightweight, non‑biodegradable, and often end up as litter, harming marine life and creating visual disamenity. The charge was set at a modest level to change behaviour without imposing a heavy financial burden.
为问题提供背景：一次性塑料袋轻薄、不可生物降解，常变成垃圾，危害海洋生物并造成视觉污染。收费被设定在温和的水平，旨在改变行为，又不至构成过重的经济负担。
2. Pinpointing the Economic Issue | 定位经济问题
The central concept here is negative externality in consumption. When consumers use plastic bags, they create external costs—litter, pollution, harm to wildlife—that are not reflected in the price of the bag.
这里的核心概念是消费中的负外部性。消费者使用塑料袋时，产生了未反映在袋子价格中的外部成本——垃圾、污染、对野生动物的伤害。
In a free market, the equilibrium quantity (Q_market) exceeds the socially optimal quantity (Q_social) because private consumers ignore these external costs. The market fails to allocate resources efficiently.
在自由市场中，均衡数量（Q_market）大于社会最优数量（Q_social），因为私人消费者忽略了这些外部成本。市场未能有效配置资源。
The case study also touches on information failure—consumers may underestimate the environmental damage—and the concept of a merit good; reducing plastic bag use generates positive externalities for society.
该案例还涉及信息失灵——消费者可能低估了环境损害——以及优值品概念；减少塑料袋使用能为社会带来正外部性。
3. Demand and Supply Analysis | 需求与供给分析
Treat the market for single‑use plastic bags at large retailers. Initially, the supply curve is perfectly elastic at a price of zero (bags are given for free). The demand curve slopes downward; consumers demand many bags when the price is zero.
考虑大型零售店中一次性塑料袋市场。起初，供给曲线在价格为零时完全有弹性（袋子免费提供）。需求曲线向下倾斜；价格为零时消费者需求很多袋子。
Imposing a 5p charge shifts the effective supply curve upward by the amount of the charge. This is equivalent to a per‑unit tax. The new equilibrium shows a higher price to consumers and a large contraction in quantity demanded.
征收5便士费用使有效供给曲线向上平移，幅度等于收费额。这相当于从量税。新均衡显示出消费者面对的价格上升，而需求量大幅收缩。
Draw a simple diagram: label the initial equilibrium at Q₀ = 140 bags per person, P = 0. After the charge, the supply curve moves to S + tax; the new equilibrium is Q₁ = 1.8 bags, P = 5p. Shade the area representing government revenue.
画一张简图：标注初始均衡在Q₀ = 人均140个袋子，P=0。收费后，供给曲线移至S + tax；新均衡在Q₁ = 1.8个袋子，P=5p。涂出代表政府收入的区域。
| Indicator | Before charge | After charge |
|---|---|---|
| Single‑use bags per person/year | 140 | 1.8 |
| Price to consumer | £0.00 | £0.05 |
| Revenue to good causes | £0 | £29m (year 1) |
4. Applying Elasticity Concepts | 运用弹性概念
The dramatic fall in quantity—from 140 to 1.8—implies a highly elastic demand. Price elasticity of demand (PED) is likely well above 1 because consumers have readily available substitutes: reusable bags, bags for life, or simply carrying items by hand.
使用量从140个骤降至1.8个，表明需求高度弹性。需求价格弹性（PED）很可能远大于1，因为消费者有现成的替代品：环保袋、可重复使用的袋子，或者索性用手拿。
The proportion of income spent on a 5p bag is extremely small, yet the behavioural ‘nudge’ effect amplifies the responsiveness. This shows that for habit‑based goods, elasticity can be significant even at low prices.
5便士塑料袋在收入中的占比极小，但行为“助推”效应放大了反应程度。这表明，对于习惯性商品，即便在低价位，弹性也可以很大。
Consider supply elasticity: large retailers can easily switch to selling thicker bags or remove free bags from checkout points. Supply is therefore relatively elastic, allowing a quick adjustment.
考虑供给弹性：大型零售商可以轻松转售更厚的袋子，或将免费袋子从收银台撤走。因此供给相对富有弹性，能迅速作出调整。
Elasticity also explains the distribution of the tax burden. With elastic demand and elastic supply, consumers bear only a small share of the economic burden; the main effect is a quantity reduction rather than a large rise in price.
弹性还解释了税负的分布。需求弹性大而供给也富有弹性时，消费者只承担一小部分经济负担；主要效果是数量减少，而非价格大幅上涨。
5. Market Failure and Externalities | 市场失灵与外部性
The market fails because marginal private cost (MPC) of using a plastic bag is close to zero for the consumer, but marginal social cost (MSC) includes litter clean‑up, harm to ecosystems, and carbon emissions from production. MSC lies above MPC.
市场失灵是因为消费者使用塑料袋的边际私人成本几乎为零，而边际社会成本包括垃圾清理、生态系统损害以及生产过程中的碳排放。MSC高于MPC。
The externality can be illustrated with a diagram: an upward‑sloping MPC curve (reflecting very low costs) and a higher MSC curve. The vertical distance between the two is the marginal external cost. The free‑market output exceeds the social optimum.
外部性可通过图形展示：向上倾斜的MPC曲线（反映极低的成本）与更高的MSC曲线。两者之间的垂直距离即边际外部成本。自由市场产出超过社会最优水平。
This is a negative consumption externality, but because the product is ‘free’ at the point of use, it also resembles a negative production externality in the disposal stage. A full analysis could mention both.
这属于负消费外部性，但由于产品在使用环节“免费”，它在处置阶段也类似于负生产外部性。完整分析可同时提及两个方面。
Government intervention aims to internalise this externality so that the price consumers pay better reflects the true social cost, moving the market towards allocative efficiency.
政府干预旨在内部化这一外部性，使消费者支付的价格更好反映真实社会成本，推动市场走向配置效率。
6. Government Intervention – The Bag Charge | 政府干预——塑料袋收费
The 5p charge is a form of Pigouvian tax set equal to the marginal external cost at the socially efficient output. In theory, it shifts the private cost curve up to coincide with MSC.
5便士收费是一种庇古税，设定在社会最优产出水平下的边际外部成本。理论上，它使私人成本曲线上移，与MSC重合。
In practice, the tax rate may not perfectly match the external cost, but the outcome—a sharp decline in bag use—suggests the charge exceeds the minimum needed to change behaviour. It also raises revenue that is ring‑fenced for environmental or charitable purposes.
实践中，税率可能并非恰好等于外部成本，但结果——塑料袋使用量急剧下降——表明收费超过了改变行为所需的最低门槛。它还筹集了被定向用于环保或慈善目的的收入。
Retailers with fewer than 250 employees are exempt. This exemption limits administrative burdens on small businesses but creates a two‑tier system, potentially reducing overall environmental gains.
员工少于250人的零售商被豁免。这一豁免减小了小企业的行政负担，但形成了双轨制，可能削弱整体环境收益。
Other instruments, such as a complete ban or a biodegradable‑bag mandate, could also be compared. The charge is a market‑based approach, giving consumers the freedom to choose while altering incentives.
其他工具如完全禁止或强制使用可降解袋也可作比较。收费属于市场导向手段，赋予消费者选择自由，同时改变激励。
7. Constructing Effective Diagrams | 绘制有效图示
In the exam, draw a side‑by‑side diagram: a demand‑supply graph on the left showing the tax effect, and a negative externality diagram on the right showing the welfare gain.
考试中，绘制并排图示：左边为显示税收效应的供需图，右边为显示福利增益的负外部性图。
Left diagram: label D, S, S+tax. Show initial equilibrium at Q₀, P=0 and new equilibrium at Q₁, P=5p. Shade the tax revenue rectangle. Annotate ‘fall in consumption’.
左图：标注D、S、S+tax。显示初始均衡Q₀、P=0及新均衡Q₁、P=5p。涂色表示税收收入矩形。标注“消费下降”。
Right diagram: draw MPC = S (private), MSC above MPC. Mark Q_market and Q_social. Use arrows to show over‑consumption. After the tax, the MPC curve shifts up, reducing quantity to Q_social and eliminating the deadweight welfare loss triangle.
右图：画出MPC = S（私人）、高于MPC的MSC。标注Q_market与Q_social。用箭头显示过度消费。征税后，MPC曲线上移，数量减少至Q_social，消除无谓福利损失三角形。
Ensure axes are labelled: ‘Price (pence)’ and ‘Quantity of bags (per year)’. A brief written explanation of each diagram must accompany it to earn full marks.
确保坐标轴标注：“价格（便士）”和“袋子数量（每年）”。每幅图都需配简短文字说明，才能拿到满分。
8. Critical Evaluation | 批判性评估
Effectiveness: The charge cut consumption by over 95% in its first year, meeting the primary goal. However, longer‑term studies show a slight rebound as consumers become desensitised to the 5p cost. The policy may need to be raised to 10p.
有效性：收费第一年就将消费削减了95%以上，达到首要目标。然而，长期研究显示，随着消费者对5便士费用不再敏感，使用量有所反弹。或许需要提高至10便士。
Equity concerns: the 5p charge is regressive; it takes a larger proportion of income from low‑income households, especially those who forget reusable bags. However, the amount is extremely small, and the proceeds fund community projects that benefit lower‑income groups.
公平性担忧：5便士收费具有累退性；对低收入家庭而言，费用占收入比例更高，尤其是忘记带环保袋的群体。不过金额极小，且收入资助了惠及低收入群体的社区项目。
Substitution effect: some consumers switched to thicker ‘bags for life’, which have a higher carbon footprint unless reused many times. This unintended consequence can partially offset environmental gains.
替代效应：部分消费者转而使用更厚的“可终身使用袋子”，这些袋子若未多次使用，碳足迹反而更高。这一意外后果可能部分抵消环境收益。
Implementation and compliance: large retailers have near‑universal compliance, but small shops remain exempt, allowing a leakage of single‑use bags. Tightening the exemption would improve effectiveness but raise costs.
实施与合规：大型零售商几乎全面遵守规定，但小商店仍属豁免范围，导致一次性塑料袋有泄漏。收紧豁免条件会提升效果，但推高成本。
Alternatives: a complete ban could be simpler to enforce but restricts consumer choice. Education campaigns or nudge policies (e.g., placing reusable bags at eye level) could complement the charge.
替代方案：全面禁止可能更易于执行，但会限制消费者选择。宣传教育或助推政策（如将环保袋放在显眼位置）可作为收费的补充。
9. Common Mistakes to Avoid | 常见错误避免
Do not confuse the 5p charge with a price floor. It is a tax, not a minimum price. Also, avoid labelling the charge as an ‘increase in cost of production’ alone—it is a per‑unit tax on the transaction.
不要把5便士收费与价格下限混淆。它是一种税收，而非最低价格。同时，避免仅将收费标为“生产成本上升”——它是对交易的从量税。
Many students analyse a negative production externality when, in fact, the primary failure is on the consumption side. Read the case carefully to determine which activity generates the external cost.
许多学生分析的是负生产外部性，而实际上主要失灵在消费端。仔细阅读案例，判断哪一活动产生了外部成本。
Avoid generic evaluation—link each evaluative point directly to the evidence in the case. For example, reference the actual drop from 140 to 1.8 bags rather than simply saying ‘demand fell’.
避免泛泛的评价——将每个评价要点直接与案例中的证据挂钩。例如，引述实际从140降至1.8个的数据，而不是简单说“需求下降”。
Diagram errors: forgetting to label curves or failing to show the shift after the tax. Remember that the vertical distance between S and S+tax must equal the tax per unit.
图示错误：忘记标注曲线或未能显示征税后的移动。记住S与S+tax之间的垂直距离必须等于单位税额。
10. Final Tips and Practice | 最终提示与练习
Structure your answer in four clear sections: definition of market failure, analysis of the externality, explanation of the intervention, and a balanced evaluation with two short‑run and two long‑run effects.
将答案清晰分为四部分：市场失灵的定义、外部性分析、干预措施说明，以及包含两个短期效应和两个长期效应的均衡性评估。
Use the chain of reasoning: start with the policy, link to price mechanism, then show how quantity and welfare change. Tools like mnemonic DEED (Define, Explain, Example, Diagram) can keep your answer focused.
使用推理链条：从政策出发，联系价格机制，再展示数量与福利的变化。利用助记符DEED（定义、解释、举例、图示）可保持答案聚焦。
For practice, swap the context—analyse a sugar tax, congestion charge, or carbon tax on petrol using the same framework. Identify similarities and differences in elasticity and stakeholder effects.
为了练习，可变换情境——运用同一框架分析糖税、拥堵费或汽油碳税。比较在弹性和利益相关者影响方面的异同。
Time yourself: allocate 25 minutes for a 12‑mark case study. Spend 5 minutes reading, 5 minutes planning and drawing diagrams, and 15 minutes writing the evaluation‑rich response.
计时练习：为一则12分案例分析分配25分钟。5分钟审题，5分钟规划和画图，15分钟撰写富含评估的答案。
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