The Concept of Elasticity | 弹性概念

📚 The Concept of Elasticity | 弹性概念

Elasticity measures how strongly one economic variable responds to a change in another. In A-Level Economics, this concept is applied mainly to demand and supply, giving students a practical toolkit for analysing markets, consumer behaviour, firm pricing and government policy.

弹性衡量一个经济变量对另一个变量变化的反应程度。在 A-Level 经济学中，弹性主要用于需求和供给分析，为学生提供分析市场、消费者行为、企业定价和政府政策的实用工具。

1. What Is Elasticity? | 什么是弹性？

In economics, elasticity is a numerical measure of responsiveness. It is calculated as the percentage change in one variable divided by the percentage change in another variable. The sign and size of the coefficient matter because they tell us both the direction and the strength of a relationship.

在经济学中，弹性是对反应程度的数值衡量。它等于一个变量变化的百分比除以另一个变量变化的百分比。系数的符号和大小都很重要，因为它们告诉我们关系的方向和强度。

Elasticity = %Δ dependent variable ÷ %Δ independent variable

弹性 = 因变量变化百分比 ÷ 自变量变化百分比

Elasticity is unit-free because it uses percentage changes. This means we can compare the responsiveness of different goods, markets or countries even when prices and quantities are measured in different units.

弹性没有单位，因为它使用百分比变化。这意味着即使价格和数量用不同单位衡量，我们也可以比较不同商品、市场或国家的反应程度。

2. Price Elasticity of Demand: Definition and Formula | 需求价格弹性：定义与公式

Price elasticity of demand (PED) measures the responsiveness of quantity demanded to a change in the good’s own price. It is the standard starting point for elasticity analysis in Cambridge A-Level Economics.

需求价格弹性 (PED) 衡量需求量对商品自身价格变化的反应程度。它是剑桥 A-Level 经济学中弹性分析的标准起点。

PED = %Δ quantity demanded ÷ %Δ price

需求价格弹性 = 需求量变化百分比 ÷ 价格变化百分比

Because price and quantity demanded usually move in opposite directions, PED is normally negative. Economists often ignore the minus sign and quote the absolute value, so a PED of -2 is described as having an elasticity of 2.

由于价格和需求量通常朝相反方向变动，PED 通常为负值。经济学家常常忽略负号并使用绝对值，因此 PED 为 -2 时会被描述为弹性等于 2。

When calculating PED, always use the percentage change formula: (new value – original value) ÷ original value × 100. In exam answers, showing the working is essential for method marks.

计算 PED 时，始终使用百分比变化公式：(新值 – 原值) ÷ 原值 × 100。在考试答案中，展示计算过程对于获得方法分至关重要。

3. Interpreting PED Values | 解读 PED 数值

\|PED\|	Description	Meaning
> 1	Elastic	Quantity demanded responds more than proportionately to a price change
= 1	Unit elastic	Quantity demanded changes by the same percentage as price
< 1	Inelastic	Quantity demanded responds less than proportionately to a price change
= 0	Perfectly inelastic	Quantity demanded does not change when price changes
∞	Perfectly elastic	Any price rise reduces quantity demanded to zero

These categories matter because they determine how total revenue responds to price changes. They also help explain why governments tax goods such as cigarettes differently from luxury yachts.

这些分类很重要，因为它们决定了总收益如何响应价格变化。它们也有助于解释为什么政府对香烟等商品征税的方式与对豪华游艇不同。

A common mistake is to describe PED as ‘elastic’ when the coefficient is greater than zero but less than one. Remember that elasticity is about the absolute value, so a PED of -0.4 is inelastic, not elastic.

一个常见错误是当系数大于零但小于一时将其描述为“有弹性”。请记住，弹性看绝对值，因此 PED 为 -0.4 是缺乏弹性，而不是富有弹性。

4. PED and Total Revenue | 需求价格弹性与总收益

Total revenue equals price multiplied by quantity sold. The effect of a price change on total revenue depends on the PED of the product.

总收益等于价格乘以销售量。价格变化对总收益的影响取决于产品的需求价格弹性。

If demand is elastic, a price cut raises total revenue because the percentage rise in quantity demanded exceeds the percentage fall in price. If demand is inelastic, a price cut lowers total revenue because quantity demanded rises only slightly.

如果需求富有弹性，降价会提高总收益，因为需求量上升的百分比超过价格下降的百分比。如果需求缺乏弹性，降价会降低总收益，因为需求量仅小幅上升。

Demand	Price rise	Price fall
Elastic	Total revenue falls	Total revenue rises
Inelastic	Total revenue rises	Total revenue falls

This relationship is frequently tested. Students should be able to explain both the arithmetic and the intuition behind the revenue effect, not just memorise the table.

这一关系经常被考查。学生应能解释收益效应背后的算术和直觉，而不仅仅是记住表格。

5. Determinants of PED | 需求价格弹性的决定因素

Several factors determine whether demand for a product is elastic or inelastic. The most important is the availability of close substitutes. Goods with many close substitutes, such as branded breakfast cereals, tend to have elastic demand because consumers can easily switch to a rival product.

有几个因素决定一种产品的需求是有弹性还是缺乏弹性。最重要的是相近替代品的可获得性。拥有许多相近替代品的商品，如品牌早餐麦片，往往需求富有弹性，因为消费者可以轻松转向竞争对手的产品。

The proportion of income spent on the good also matters. Goods that take a large share of a consumer’s budget, such as cars or foreign holidays, tend to have more elastic demand than inexpensive items like salt or matches.

商品支出占收入的比例也很重要。占消费者预算较大份额的商品，如汽车或海外度假，往往比盐或火柴等廉价商品的需求更富有弹性。

Necessities generally have inelastic demand, while luxuries have elastic demand. Addictive goods, such as cigarettes, also have inelastic demand because consumers find it difficult to reduce consumption even when price rises.

必需品通常需求缺乏弹性，而奢侈品需求富有弹性。成瘾性商品，如香烟，也缺乏弹性，因为即使价格上涨，消费者也难以减少消费。

The time period is crucial: demand is usually more elastic in the long run because consumers have more time to find substitutes and adjust their behaviour.

时间周期也至关重要：长期中需求通常更富有弹性，因为消费者有更多时间寻找替代品并调整行为。

6. Income Elasticity of Demand | 需求收入弹性

Income elasticity of demand (YED) measures how quantity demanded responds to a change in consumer income. It is calculated as the percentage change in quantity demanded divided by the percentage change in income.

需求收入弹性 (YED) 衡量需求量如何响应消费者收入的变化。它等于需求量变化百分比除以收入变化百分比。

YED = %Δ quantity demanded ÷ %Δ income

需求收入弹性 = 需求量变化百分比 ÷ 收入变化百分比

Normal goods have a positive YED: as income rises, demand rises. Inferior goods have a negative YED: as income rises, demand falls. Within normal goods, necessities have a YED between 0 and 1, while luxuries have a YED greater than 1.

正常商品的 YED 为正：收入上升，需求上升。低档商品的 YED 为负：收入上升，需求下降。在正常商品中，必需品的 YED 在 0 到 1 之间，而奢侈品的 YED 大于 1。

YED is useful for firms forecasting demand as the economy grows or contracts. It also explains why some industries expand rapidly during booms while others, such as producers of budget goods, may see a fall in demand.

YED 有助于企业在经济增长或收缩时预测需求。它还可以解释为什么一些行业在繁荣时期迅速扩张，而另一些行业，如平价商品生产商，可能会看到需求下降。

7. Cross Elasticity of Demand | 需求交叉弹性

Cross elasticity of demand (XED) measures how the quantity demanded of one good responds to a change in the price of another good. It is calculated as the percentage change in quantity demanded of good A divided by the percentage change in price of good B.

需求交叉弹性 (XED) 衡量一种商品的需求量如何响应另一种商品价格的变化。它等于商品 A 需求量变化百分比除以商品 B 价格变化百分比。

XED = %Δ quantity demanded of good A ÷ %Δ price of good B

需求交叉弹性 = 商品 A 需求量变化百分比 ÷ 商品 B 价格变化百分比

Substitutes have a positive XED because a rise in the price of one good increases demand for the other. Complements have a negative XED because a rise in the price of one good reduces demand for the other. Unrelated goods have an XED close to zero.

替代品的 XED 为正，因为一种商品价格上涨会增加另一种商品的需求。互补品的 XED 为负，因为一种商品价格上涨会减少另一种商品的需求。无关商品的 XED 接近零。

The size of the XED coefficient indicates the closeness of the relationship. For example, a strongly positive XED between two brands of coffee suggests they are close substitutes, which is useful information for firms considering a price change.

XED 系数的大小表明关系的密切程度。例如，两个咖啡品牌之间的 XED 为强正值，表明它们是相近的替代品，这对考虑调价的企业来说是有用的信息。

8. Price Elasticity of Supply | 供给价格弹性

Price elasticity of supply (PES) measures how quantity supplied responds to a change in price. Unlike PED, PES is usually positive because higher prices give producers an incentive to supply more.

供给价格弹性 (PES) 衡量供给量如何响应价格变化。与 PED 不同，PES 通常为正值，因为更高的价格激励生产者增加供给。

PES = %Δ quantity supplied ÷ %Δ price

供给价格弹性 = 供给量变化百分比 ÷ 价格变化百分比

If PES is greater than 1, supply is elastic and producers can increase output quickly without a large rise in cost. If PES is less than 1, supply is inelastic and output is difficult to expand in the short run.

如果 PES 大于 1，供给富有弹性，生产者可以快速增加产量而成本不会大幅上升。如果 PES 小于 1，供给缺乏弹性，短期内产量难以扩大。

Supply tends to be more elastic in the long run than in the short run because firms can build new factories, hire more workers and adopt new technology over time.

长期中供给往往比短期更富有弹性，因为企业可以随着时间推移建设新工厂、雇用更多工人并采用新技术。

9. Determinants of PES | 供给价格弹性的决定因素

The main determinant of PES is spare capacity. If firms are operating below full capacity, they can increase output quickly in response to a price rise, making supply elastic. If they are already at full capacity, supply is inelastic.

PES 的主要决定因素是闲置产能。如果企业未满负荷运转，它们可以在价格上升时快速增加产出，使供给富有弹性。如果企业已经满负荷运转，供给则缺乏弹性。

The level of stocks also matters. Firms holding large inventories can release them onto the market quickly, increasing PES. Perishable goods, which cannot be stored for long, often have inelastic supply.

库存水平也很重要。持有大量库存的企业可以迅速将产品投放市场，从而提高 PES。易腐商品不能长期储存，因此供给往往缺乏弹性。

Ease of switching production is another factor. If resources can be moved from one product to another without much cost or delay, supply is more elastic. Complex production processes with specialised machinery tend to make supply inelastic.

转换生产的难易程度是另一个因素。如果资源可以在不产生太多成本或延迟的情况下从一种产品转移到另一种产品，供给就更富有弹性。需要专用机械的复杂生产过程往往使供给缺乏弹性。

Time is the most universal determinant: the longer the adjustment period, the more elastic supply becomes.

时间是最普遍的决定因素：调整期越长，供给越富有弹性。

10. Applications: Tax Incidence and Business Strategy | 应用：税收归宿与商业策略

Elasticity is central to understanding who bears the burden of an indirect tax. When demand is inelastic and supply is elastic, consumers bear most of the tax because they continue buying even when the price rises. When demand is elastic and supply is inelastic, producers bear most of the burden.

弹性对于理解间接税的负担由谁承担至关重要。当需求缺乏弹性而供给富有弹性时，消费者承担大部分税收，因为即使价格上涨他们仍继续购买。当需求富有弹性而供给缺乏弹性时，生产者承担大部分负担。

Governments exploit this when taxing demerit goods such as tobacco and alcohol. Because demand is inelastic, tax revenue is high and consumption falls only modestly, which is often the intended policy outcome.

政府在征税于烟草和酒精等有害商品时利用了这一点。由于需求缺乏弹性，税收收入高且消费仅小幅下降，这通常是政策预期的结果。

Firms also use elasticity when setting prices. A business with elastic demand should avoid raising prices, while a business with inelastic demand can raise prices to increase revenue. Elasticity estimates help managers make better pricing decisions.

企业在定价时也会使用弹性。需求富有弹性的企业应避免提价，而需求缺乏弹性的企业可以通过提价来增加收益。弹性估算有助于管理者做出更优的定价决策。

11. Common Exam Mistakes | 常见考试误区

One frequent mistake is confusing PED with YED. PED is about the good’s own price, while YED is about consumer income. Make sure the formula matches the type of elasticity being discussed.

一个常见错误是混淆 PED 和 YED。PED 关乎商品自身价格，而 YED 关乎消费者收入。确保公式与所讨论的弹性类型匹配。

Another error is forgetting the sign conventions. PED is usually negative, YED can be positive or negative, and XED can be positive, negative or zero. The sign carries economic meaning and should not be dropped casually.

另一个错误是忽略符号规则。PED 通常为负，YED 可为正或负，XED 可为正、负或零。符号具有经济含义，不应随意省略。

Students also sometimes calculate elasticity using absolute changes rather than percentage changes. Since elasticity is unit-free, percentage changes are essential for correct comparison.

学生有时会用绝对变化而非百分比变化来计算弹性。由于弹性没有单位，百分比变化对于正确比较至关重要。

Finally, in essay questions, do not simply state the elasticity value. Explain why it matters for total revenue, tax incidence or business decisions. Application earns high marks at A-Level.

最后，在论述题中，不要只陈述弹性数值。要解释它为什么对总收益、税收归宿或商业决策重要。在 A-Level 中，应用分析才能获得高分。

12. Summary Table | 总结表

Type	Formula	Sign	Key use
PED	%ΔQd ÷ %ΔP	Usually negative	Pricing and total revenue
YED	%ΔQd ÷ %ΔIncome	Positive or negative	Normal vs inferior goods
XED	%ΔQd of A ÷ %ΔP of B	Positive, negative or zero	Substitutes and complements
PES	%ΔQs ÷ %ΔP	Usually positive	Supply responsiveness and time

Mastering these four elasticities gives you a powerful framework for evaluating real-world markets. In Cambridge A-Level Economics, both calculation and evaluation skills are tested, so practise applying the concept to different scenarios.

掌握这四种弹性为你评估现实世界市场提供了一个强大的框架。在剑桥 A-Level 经济学中，计算和评估能力都会考查，因此要练习将弹性概念应用于不同情境。

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The Concept of Elasticity | 弹性概念