Elasticity in IGCSE OCR Economics | 弹性考点精讲

📚 Elasticity in IGCSE OCR Economics | 弹性考点精讲

Elasticity measures how responsive one variable is to a change in another. In IGCSE OCR Economics, mastering elasticity is essential for understanding market behaviour, pricing strategies and the effects of government policies. This article covers all the key elasticity concepts you need for the exam, including price elasticity of demand and supply, income elasticity and cross elasticity, with clear definitions, calculations and real-world examples.

弹性衡量一个变量对另一个变量变化的反应程度。在 IGCSE OCR 经济学中，掌握弹性对于理解市场行为、定价策略以及政府政策的影响至关重要。本文涵盖你考试所需的所有关键弹性概念，包括需求价格弹性、供给价格弹性、收入弹性和交叉弹性，并提供清晰的定义、计算方法和现实案例。

1. What is Elasticity? | 什么是弹性？

Elasticity is a numerical measure of responsiveness. In economics, it quantifies how much the quantity demanded or supplied of a good changes when one of its determinants (such as price or income) changes. The sign and magnitude of the elasticity coefficient tell us the direction and strength of the relationship.

弹性是反应程度的数值度量。在经济学中，它量化了一种商品的需求量或供给量在其某个决定因素（如价格或收入）变化时的变化程度。弹性系数的符号和大小告诉我们关系的方向和强度。

2. Price Elasticity of Demand (PED) – Definition and Formula | 需求价格弹性（PED）——定义与公式

Price elasticity of demand measures the responsiveness of quantity demanded to a change in the good’s own price. It is calculated as the percentage change in quantity demanded divided by the percentage change in price.

需求价格弹性衡量需求量对商品自身价格变化的反应程度。它的计算方法是需求量变化的百分比除以价格变化的百分比。

PED = %ΔQd / %ΔP

Since price and quantity demanded usually move in opposite directions (the law of demand), PED is typically negative. OCR often uses the absolute value, ignoring the minus sign, so a PED of -0.5 is treated as 0.5.

由于价格和需求量通常朝相反方向变动（需求定律），PED 通常为负值。OCR 经济学通常使用绝对值，忽略负号，因此 PED 为 -0.5 视为 0.5。

3. Interpreting PED Values | 解读 PED 值

PED Value	Classification	Meaning
PED = 0	Perfectly inelastic	Quantity demanded does not change when price changes.
0 < PED < 1	Inelastic	% change in Qd is smaller than % change in P. Consumers are not very responsive.
PED = 1	Unit elastic	% change in Qd equals % change in P. Total revenue stays the same.
1 < PED < ∞	Elastic	% change in Qd is larger than % change in P. Consumers are very responsive.
PED = ∞	Perfectly elastic	Any price increase causes quantity demanded to fall to zero.

中文翻译：PED=0 完全无弹性；0

4. PED and Total Revenue | PED 与总收益

Total revenue (TR) equals price multiplied by quantity sold. The relationship between PED and TR is a key exam topic. For an elastic good (PED > 1), a price cut raises TR because the percentage increase in quantity sold outweighs the percentage fall in price. Conversely, a price rise reduces TR.

总收益 (TR) 等于价格乘以销售量。PED 与 TR 之间的关系是重要考点。对于弹性商品 (PED > 1)，降价会提高总收益，因为销售量增加的百分比超过了价格下降的百分比。相反，涨价会减少总收益。

For an inelastic good (PED < 1), a price rise increases TR, as the fall in quantity sold is proportionally smaller. A price cut reduces TR. For unit elastic demand (PED = 1), changes in price leave total revenue unchanged.

对于缺乏弹性的商品 (PED < 1)，涨价会增加总收益，因为销售量减少的比例较小。降价则会减少总收益。对于单位弹性需求 (PED = 1)，价格变动不会改变总收益。

5. Determinants of PED | PED 的影响因素

Several factors make demand more elastic or inelastic:

多种因素会使需求变得更有弹性或更缺乏弹性：

Availability of close substitutes – more substitutes make demand more elastic. | 相近替代品的可获得性——替代品越多，需求弹性越大。
Necessity vs luxury – necessities tend to be inelastic; luxuries are elastic. | 必需品与奢侈品——必需品往往缺乏弹性；奢侈品富有弹性。
Proportion of income spent – goods that take a large share of income have more elastic demand. | 支出占收入的比例——占收入比重大的商品，需求更有弹性。
Time period – demand is usually more elastic in the long run as consumers can find alternatives. | 时间周期——长期内需求通常更有弹性，因为消费者可以找到替代品。
Addiction and habit – goods like cigarettes have inelastic demand. | 成瘾与习惯——香烟等商品需求缺乏弹性。
Definition of the market – narrowly defined markets (e.g. a specific brand) have more elastic demand than broadly defined markets (e.g. food). | 市场的定义——狭义市场（如某一品牌）比广义市场（如食品）的需求更具弹性。

6. Price Elasticity of Supply (PES) – Definition and Formula | 供给价格弹性（PES）——定义与公式

Price elasticity of supply measures the responsiveness of quantity supplied to a change in price. It is calculated as the percentage change in quantity supplied divided by the percentage change in price.

供给价格弹性衡量供给量对价格变化的反应程度。它的计算方法是供给量变化的百分比除以价格变化的百分比。

PES = %ΔQs / %ΔP

PES is usually positive because price and quantity supplied move in the same direction. A value greater than 1 indicates elastic supply; less than 1 indicates inelastic supply. PES = 1 is unit elastic, PES = 0 is perfectly inelastic, and PES = ∞ is perfectly elastic.

PES 通常为正值，因为价格和供给量同方向变动。大于 1 表示供给富有弹性；小于 1 表示供给缺乏弹性。PES = 1 单位弹性，PES = 0 完全无弹性，PES = ∞ 完全弹性。

7. Determinants of PES | PES 的影响因素

The key factor is the flexibility of producers to change output. Factors that influence PES include:

关键因素是生产者改变产量的灵活性。影响 PES 的因素包括：

Time period – supply is very inelastic in the momentary period, more elastic in the short run, and most elastic in the long run. | 时间周期——瞬期中供给极缺乏弹性，短期更具弹性，长期弹性最大。
Spare production capacity – if a firm has unused capacity, supply can be elastic. | 闲置产能——如果企业有未利用的产能，供给可以具有弹性。
Ease of storing stock – goods that can be stored easily tend to have more elastic supply. | 库存储存的难易程度——易于储存的商品供给往往更具弹性。
Complexity of production – goods requiring specialised labour or long production times have inelastic supply. | 生产的复杂性——需要专业劳动力或较长生产时间的商品供给缺乏弹性。
Availability of raw materials – easy access to inputs makes supply more elastic. | 原材料的可获得性——易于获取投入品会使供给更具弹性。

8. Income Elasticity of Demand (YED) | 需求收入弹性（YED）

Income elasticity of demand measures how quantity demanded responds to a change in consumers’ income. The formula is:

需求收入弹性衡量需求量如何随消费者收入的变化而变化。其公式为：

YED = %ΔQd / %ΔY

Where Y represents income. YED can be positive or negative:

其中 Y 代表收入。YED 可为正或负：

Positive YED: The good is a normal good. If YED > 1, it is a luxury good (income elastic). If 0 < YED < 1, it is a necessity (income inelastic).
负值 YED: 商品是劣等品 (inferior good)。收入增加时，需求量减少。
正值 YED: 商品是正常品。若 YED > 1，为奢侈品（收入弹性）；若 0 < YED < 1，为必需品（收入缺乏弹性）。

Examples: foreign holidays (luxury, YED > 1), basic food (necessity, 0 < YED < 1), cheap no-brand crisps (inferior, YED < 0).

例如：出国度假（奢侈品，YED > 1），基本食物（必需品，0 < YED < 1），廉价无品牌薯片（劣等品，YED < 0）。

9. Cross Elasticity of Demand (XED) | 需求交叉弹性（XED）

Cross elasticity of demand measures the responsiveness of demand for one good (Good A) to a change in the price of another good (Good B).

需求交叉弹性衡量一种商品（商品 A）的需求对另一种商品（商品 B）价格变化的反应程度。

XED = %ΔQd of Good A / %ΔP of Good B

The sign of XED reveals the relationship between the goods:

XED 的符号揭示了商品之间的关系：

Positive XED: The goods are substitutes. A rise in the price of B increases demand for A. | 正值 XED：商品互为替代品。B 的价格上升会增加对 A 的需求。
Negative XED: The goods are complements. A rise in the price of B reduces demand for A. | 负值 XED：商品互为互补品。B 的价格上升会减少对 A 的需求。
XED close to 0: The goods are unrelated. | XED 接近 0：商品不相关。

The magnitude indicates the strength of the relationship: a larger absolute value means a stronger substitute or complement relationship. XED is vital for firms when considering competitors’ pricing and joint product strategies.

数值的大小表示关系的强度：绝对值越大，替代或互补关系越强。XED 对企业考虑竞争对手定价和联合产品策略至关重要。

10. Exam Tips and Common Pitfalls | 考试技巧与常见误区

When calculating elasticity, always use percentage changes, not absolute changes. Remember the midpoint formula may be required for greater accuracy, but OCR usually accepts the simple percentage change method. Always check your sign: PED is negative, PES is positive, and YED/XED signs have economic meaning. Avoid confusing movement along a curve (change in price, effect on Qd/Qs) with shifts caused by other determinants.

计算弹性时，务必使用百分比变化，而非绝对变化。记住中点公式可提高准确性，但 OCR 通常接受简单的百分比变化法。始终检查符号：PED 为负，PES 为正，YED/XED 的符号具有经济含义。避免混淆沿着曲线的移动（价格变化对 Qd/Qs 的影响）与其他决定因素引起的移动。

In long-answer questions, link elasticity to real-world applications: taxation (inelastic demand means more tax burden on consumers), subsidies, price volatility in agricultural markets (inelastic supply and demand), and business pricing decisions. Use diagrams where appropriate, showing steep or flat demand curves to illustrate elastic and inelastic demand.

在长篇问答中，将弹性与现实应用联系起来：税收（需求缺乏弹性意味着消费者承担更多税负）、补贴、农产品市场的价格波动（供给和需求缺乏弹性）以及企业定价决策。在适当的地方使用图表，显示陡峭或平坦的需求曲线来说明弹性和缺乏弹性。

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Elasticity in IGCSE OCR Economics | 弹性 考点精讲