A-Level Economics: Price Elasticity of Demand and Supply — Complete Guide | A-Level经济：需求与供给的价格弹性完全指南

Introduction to Elasticity | 弹性概念导论

Elasticity is one of the most fundamental and frequently examined concepts in A-Level Economics. It measures how responsive one economic variable is to changes in another. For A-Level students (particularly those sitting CIE, Edexcel, and AQA examinations), mastering elasticity is essential — it typically accounts for 15-20% of the marks on Paper 1 and Paper 2 combined. This article provides a rigorous, exam-focused treatment of price elasticity of demand (PED), price elasticity of supply (PES), and their applications to real-world markets.

弹性是 A-Level 经济学中最基础、最常考的概念之一。它衡量一个经济变量对另一个变量变化的反应程度。对于 A-Level 学生（尤其是参加 CIE、Edexcel 和 AQA 考试的学生）来说，掌握弹性至关重要——它通常占 Paper 1 和 Paper 2 总分的 15-20%。本文将对需求价格弹性（PED）、供给价格弹性（PES）及其在现实市场中的应用进行严谨的、面向考试的讲解。

Price Elasticity of Demand (PED) | 需求价格弹性

Definition and Formula | 定义与公式

Price Elasticity of Demand (PED) measures the responsiveness of quantity demanded to a change in the price of the good itself, ceteris paribus. The formula is:

需求价格弹性（PED）衡量的是商品需求量对自身价格变化的反应程度，假设其他条件不变（ceteris paribus）。公式如下：

PED = % Change in Quantity Demanded / % Change in Price

Because price and quantity demanded move in opposite directions (the law of demand), PED is always negative. However, in A-Level Economics, we typically report the absolute value of PED, ignoring the negative sign. When examiners ask “what is the PED?”, they generally expect a positive number representing the magnitude of responsiveness.

由于价格和需求量呈反向变动（需求定律），PED 始终为负值。然而，在 A-Level 经济学中，我们通常报告 PED 的绝对值，忽略负号。当考官问”PED 是多少？”时，他们通常期望一个代表反应程度大小的正数。

Interpreting PED Values | PED 数值的解读

PED Value	Classification | 分类	Meaning | 含义
PED > 1	Price Elastic | 富有弹性	Quantity demanded changes more than proportionately to price. Consumers are very responsive to price changes.
PED = 1	Unit Elastic | 单位弹性	Quantity demanded changes exactly proportionately to price. Total revenue remains constant.
0 < PED < 1	Price Inelastic | 缺乏弹性	Quantity demanded changes less than proportionately to price. Consumers are relatively unresponsive to price changes.
PED = 0	Perfectly Inelastic | 完全无弹性	Quantity demanded does not change at all when price changes (vertical demand curve).
PED = ∞	Perfectly Elastic | 完全弹性	Any price increase causes quantity demanded to fall to zero (horizontal demand curve).

Determinants of PED | PED 的决定因素

Understanding what makes demand elastic or inelastic is crucial for both multiple-choice questions and essay answers. The key determinants are:

理解哪些因素使需求富有弹性或缺乏弹性，对于选择题和论述题都至关重要。关键决定因素如下：

• Availability of Substitutes | 替代品的可得性 — The more close substitutes a good has, the more elastic its demand. A small price increase on Coca-Cola sends consumers to Pepsi. Conversely, insulin has no close substitute, making its demand highly inelastic.
• Necessity vs. Luxury | 必需品与奢侈品 — Necessities (bread, water, electricity) tend to have inelastic demand. Luxuries (designer handbags, overseas holidays) tend to have elastic demand.
• Proportion of Income | 占收入的比例 — Goods that take up a large proportion of a consumer’s income (cars, houses) have more elastic demand. Goods that cost a small fraction (salt, matches, chewing gum) tend to be inelastic.
• Time Period | 时间跨度 — Demand is more elastic in the long run than in the short run. Consumers need time to adjust their behaviour — finding alternatives, changing habits, or switching technologies.
• Addictiveness / Habit Formation | 成瘾性 / 习惯形成 — Addictive goods like cigarettes and alcohol have inelastic demand. Habitual purchases also exhibit low elasticity.
• Definition of the Market | 市场定义范围 — Narrowly defined markets (a specific brand of cereal) have more elastic demand than broadly defined markets (food in general).

PED and Total Revenue | PED 与总收益

One of the most important exam applications of PED is its relationship with Total Revenue (TR). Total Revenue = Price × Quantity. Firms can use knowledge of PED to make pricing decisions:

PED 最重要的考试应用之一是其与总收益（TR）的关系。总收益 = 价格 × 数量。企业可以利用 PED 的知识来制定定价决策：

• If demand is elastic (PED > 1): Raising price reduces total revenue. Lowering price increases total revenue. Example: A cinema reduces ticket prices from £10 to £8, quantity demanded rises from 500 to 700. TR rises from £5,000 to £5,600.
• 如果需求富有弹性（PED > 1）：提高价格会减少总收益。降低价格会增加总收益。例如：电影院将票价从 10 英镑降至 8 英镑，需求量从 500 张增加到 700 张。TR 从 5,000 英镑增加到 5,600 英镑。
• If demand is inelastic (PED < 1): Raising price increases total revenue. Lowering price reduces total revenue. Example: A petrol station raises petrol prices by 5%, volume falls by only 2%. TR increases.
• 如果需求缺乏弹性（PED < 1）：提高价格会增加总收益。降低价格会减少总收益。例如：加油站将汽油价格提高 5%，销售量仅下降 2%。TR 增加。
• If demand is unit elastic (PED = 1): Changing price has no effect on total revenue. TR is maximised at the midpoint of a linear demand curve.
• 如果需求为单位弹性（PED = 1）：改变价格不会影响总收益。在线性需求曲线的中点处，TR 达到最大。

Price Elasticity of Supply (PES) | 供给价格弹性

Definition and Formula | 定义与公式

Price Elasticity of Supply (PES) measures the responsiveness of quantity supplied to a change in the price of the good. Unlike PED, PES is always positive — price and quantity supplied move in the same direction (the law of supply). The formula is:

供给价格弹性（PES）衡量的是商品供给量对价格变化的反应程度。与 PED 不同，PES 始终为正值——价格和供给量同方向变动（供给定律）。公式如下：

PES = % Change in Quantity Supplied / % Change in Price

Interpreting PES Values | PES 数值的解读

PES Value	Classification | 分类	Meaning | 含义
PES > 1	Price Elastic Supply | 供给富有弹性	Quantity supplied changes more than proportionately to price. Producers can easily expand output.
PES = 1	Unit Elastic Supply | 单位弹性供给	Quantity supplied changes exactly proportionately to price.
0 < PES < 1	Price Inelastic Supply | 供给缺乏弹性	Quantity supplied changes less than proportionately to price. Producers struggle to increase output quickly.
PES = 0	Perfectly Inelastic Supply | 完全无弹性供给	Quantity supplied is fixed regardless of price (vertical supply curve). Example: fixed number of stadium seats.
PES = ∞	Perfectly Elastic Supply | 完全弹性供给	Producers will supply any quantity at a given price (horizontal supply curve).

Determinants of PES | PES 的决定因素

• Time Period | 时间跨度 — The most important determinant. In the momentary period (immediate), supply is perfectly inelastic — producers cannot change output at all. In the short run, at least one factor of production is fixed, making supply relatively inelastic. In the long run, all factors are variable, making supply more elastic.
• Spare Production Capacity | 闲置产能 — Firms with unused capacity can increase output quickly and cheaply, making supply more elastic. Firms operating at full capacity face inelastic supply in the short run.
• Ease of Storage | 储存难易程度 — Goods that can be stored easily (canned food, oil) have more elastic supply because firms can release inventory when prices rise. Perishable goods (fresh produce) have inelastic supply.
• Complexity of Production | 生产复杂程度 — Goods with simple production processes (textiles, basic manufacturing) have more elastic supply. Highly complex goods (aerospace components, specialised machinery) have inelastic supply.
• Availability of Raw Materials | 原材料可得性 — If raw materials are abundant and easily sourced, supply is more elastic. Scarcity of key inputs makes supply inelastic.
• Mobility of Factors of Production | 生产要素流动性 — When labour and capital can be easily moved between industries, supply is more elastic.

Exam Technique: How to Score Top Marks | 考试技巧：如何获得高分

Common Question Types | 常见题型

1. Definition Questions (2-4 marks) — “Define price elasticity of demand.” Provide the precise definition, the formula, and one sentence explaining what the coefficient means. Always state that PED is negative but reported in absolute terms.

1. 定义题（2-4 分）——”定义需求价格弹性。”提供精确定义、公式，并用一句话解释系数的含义。务必说明 PED 为负值，但按绝对值报告。

2. Calculation Questions (4-6 marks) — “Using the data provided, calculate the PED for good X.” Show your working clearly: (1) write the formula, (2) calculate the percentage changes correctly using the midpoint formula if instructed, (3) state the answer and interpret it. The midpoint formula is: %ΔQ = (Q₂ – Q₁) / [(Q₁ + Q₂)/2] × 100.

2. 计算题（4-6 分）——”使用提供的数据，计算商品 X 的 PED。”清晰展示计算过程：(1) 写出公式，(2) 正确计算百分比变化（如要求使用中点公式），(3) 给出答案并解释。中点公式为：%ΔQ = (Q₂ – Q₁) / [(Q₁ + Q₂)/2] × 100。

3. Diagram Questions (6-8 marks) — “Draw a diagram showing perfectly inelastic demand and explain its significance.” Draw the vertical demand curve, label both axes clearly (Price on the vertical axis, Quantity on the horizontal), and explain with a real-world example like life-saving medicines.

3. 图表题（6-8 分）——”画出完全无弹性需求的示意图，并解释其意义。”画出垂直需求曲线，清晰标注两轴（纵轴为价格，横轴为数量），并用救命药物等现实例子进行解释。

4. Essay Questions (12-25 marks) — “Discuss the factors that determine the price elasticity of demand for a product of your choice.” Structure your answer: introduction defining PED, body paragraphs covering 3-4 key determinants with applied examples, and a conclusion ranking the factors in order of importance for your chosen product. Always include evaluation — acknowledge that different factors matter more in different contexts.

4. 论述题（12-25 分）——”讨论决定你所选产品需求价格弹性的因素。”结构化你的答案：引言定义 PED，主体段落涵盖 3-4 个关键决定因素并附上应用实例，结尾对你所选产品按重要性排序各因素。务必包含评估——承认不同因素在不同情境下的重要性不同。

Real-World Applications | 实际应用

Government Tax Policy | 政府税收政策

Governments use elasticity analysis when designing indirect taxes. To maximise tax revenue, a government should levy taxes on goods with inelastic demand (like petrol, alcohol, and tobacco). When demand is inelastic, consumers bear most of the tax burden (tax incidence falls mainly on consumers). This is why “sin taxes” are effective — reduced consumption is a secondary goal; revenue generation is primary.

政府在制定间接税时利用弹性分析。为了最大化税收收入，政府应对需求缺乏弹性的商品（如汽油、酒精和烟草）征税。当需求缺乏弹性时，消费者承担大部分税负（税收归宿主要落在消费者身上）。这就是”罪恶税”有效的原因——减少消费是次要目标，创收才是主要目标。

Agricultural Markets | 农产品市场

Agricultural products typically have both inelastic demand (food is a necessity) and inelastic supply in the short run (production cycles are long). This combination creates the cobweb model — where supply decisions made today, based on today’s price, lead to price fluctuations when the harvest arrives. Understanding this helps explain why governments intervene in agricultural markets with buffer stock schemes and price floors.

农产品通常同时具有缺乏弹性的需求（食品是必需品）和短期内缺乏弹性的供给（生产周期长）。这种组合产生了蛛网模型——今天基于当前价格做出的供给决策，在收成到来时导致价格波动。理解这一点有助于解释为什么政府会通过缓冲库存计划和价格下限来干预农产品市场。

Housing Market | 住房市场

The housing market demonstrates why supply elasticity matters. Housing supply is highly inelastic in the short run (planning permission, construction time, land scarcity). When demand surges — due to population growth, low interest rates, or quantitative easing — prices rise sharply because supply cannot adjust quickly. This creates housing affordability crises seen in cities like London, Hong Kong, and San Francisco. The policy solution involves both demand-side measures (stamp duty, mortgage regulation) and supply-side measures (planning reform, social housing construction).

住房市场展示了供给弹性的重要性。住房供给在短期内高度缺乏弹性（规划许可、建设时间、土地稀缺）。当需求激增——由于人口增长、低利率或量化宽松——价格急剧上涨，因为供给无法快速调整。这造成了伦敦、香港和旧金山等城市出现的住房负担能力危机。政策解决方案涉及需求侧措施（印花税、抵押贷款监管）和供给侧措施（规划改革、社会住房建设）。

Common Exam Mistakes to Avoid | 常见考试错误

1. Forgetting the negative sign interpretation: Always mention that PED is negative but state you are using the absolute value. Many marks are lost when students incorrectly treat PED as positive without explanation.
2. 混淆负号解读：务必说明 PED 为负值，但声明你使用的是绝对值。许多学生因错误地将 PED 当作正值而不加解释而失分。
3. Confusing percentage changes with absolute changes: PED uses proportional changes, not absolute units. A £1 price change has a completely different effect on a £1 good versus a £1,000 good.
4. 混淆百分比变化与绝对变化：PED 使用比例变化，而非绝对单位。1 英镑的价格变化对 1 英镑商品和 1,000 英镑商品的影响完全不同。
5. Incorrectly applying the midpoint formula: When calculating PED from two price-quantity points, use the midpoint (arc) formula unless the question specifically asks for point elasticity. The midpoint formula prevents different answers depending on the direction of the price change.
6. 中点公式应用错误：从两个价格-数量点计算 PED 时，使用中点（弧）公式，除非题目明确要求点弹性。中点公式可避免因价格变动方向不同而得到不同答案。
7. Drawing demand curves with constant elasticity: A straight-line (linear) demand curve does NOT have constant elasticity. Elasticity varies along the curve — it is elastic at high prices and inelastic at low prices. The midpoint is unit elastic.
8. 画出恒弹性需求曲线：直线（线性）需求曲线不具有恒定弹性。弹性沿曲线变化——在高价位富有弹性，在低价位缺乏弹性。中点是单位弹性。
9. Conflating PED with income elasticity or cross elasticity: PED only considers a good’s own price. Income elasticity (YED) and cross elasticity (XED) are distinct concepts tested separately.
10. 将 PED 与收入弹性或交叉弹性混淆：PED 仅考虑商品自身价格。收入弹性（YED）和交叉弹性（XED）是不同的概念，分别进行测试。

Practice Questions | 练习题目

Test your understanding with these exam-style questions:

Question 1 (4 marks): A coffee shop raises the price of a latte from £3.50 to £4.00. Daily sales fall from 200 to 170 lattes. Calculate the PED using the midpoint formula and interpret the result.

Question 2 (12 marks): Discuss the extent to which knowledge of price elasticity of demand might be useful to a firm considering a price increase for its product.

Question 3 (8 marks): Explain, using appropriate diagrams, why the price elasticity of supply for agricultural products differs between the short run and the long run.

第 1 题（4 分）：一家咖啡店将拿铁价格从 3.50 英镑提高到 4.00 英镑。日销量从 200 杯降至 170 杯。使用中点公式计算 PED 并解释结果。

第 2 题（12 分）：讨论需求价格弹性的知识在多大程度上对考虑提高产品价格的企业有用。

第 3 题（8 分）：使用适当的图表解释为什么农产品的供给价格弹性在短期和长期之间有所不同。

Key Definitions Summary | 关键定义总结

Term | 术语	Definition | 定义
Price Elasticity of Demand (PED) | 需求价格弹性	A measure of the responsiveness of quantity demanded to a change in the good’s own price, calculated as %ΔQD ÷ %ΔP.
Price Elasticity of Supply (PES) | 供给价格弹性	A measure of the responsiveness of quantity supplied to a change in the good’s price, calculated as %ΔQS ÷ %ΔP.
Total Revenue | 总收益	The total income received by firms from selling a good, calculated as Price × Quantity sold.
Tax Incidence | 税收归宿	The division of a tax burden between consumers and producers, determined by the relative elasticities of demand and supply.
Arc Elasticity | 弧弹性	Elasticity calculated between two points on a curve using the midpoint formula to give consistent results regardless of direction.
Point Elasticity | 点弹性	Elasticity measured at a single point on a demand or supply curve.
Cobweb Model | 蛛网模型	A model explaining persistent price fluctuations in markets where supply decisions are based on lagged prices.