Price Elasticity of Demand Explained | 需求的价格弹性解析

📚 Price Elasticity of Demand Explained | 需求的价格弹性解析

Price elasticity of demand (PED) is one of the most important concepts in A-Level economics. It measures how strongly the quantity demanded of a good responds to a change in its price. Understanding PED helps students analyse consumer behaviour, business pricing decisions, and government tax policy.

需求的价格弹性是 A-Level 经济学的核心概念之一，它衡量商品需求量对价格变化的反应程度。理解需求价格弹性有助于学生分析消费者行为、企业定价决策以及政府税收政策。

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1. What Is Price Elasticity of Demand? | 什么是需求的价格弹性？

Price elasticity of demand is the percentage change in quantity demanded divided by the percentage change in price. It tells us whether consumers react strongly or weakly when a firm changes the price of its product. A high value means consumers are very responsive; a low value means they are relatively unresponsive.

需求的价格弹性是指需求量变动百分比除以价格变动百分比。它反映消费者对商品价格变动的敏感程度：弹性值高，说明消费者对价格变动反应强烈；弹性值低，说明消费者对价格变动反应较弱。

For example, if the price of a luxury holiday rises by 10% and the quantity demanded falls by 30%, the PED is -3. This means demand is highly elastic. In contrast, if the price of insulin rises by 10% and quantity demanded falls by only 2%, the PED is -0.2, so demand is inelastic.

例如，如果豪华假期的价格上升 10%，需求量下降 30%，则需求价格弹性为 -3，表明需求高度有弹性。相比之下，如果胰岛素价格上升 10%，需求量仅下降 2%，则需求价格弹性为 -0.2，表明需求缺乏弹性。

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2. The PED Formula | 需求价格弹性公式

The standard formula is:

标准公式为：

PED = %ΔQd ÷ %ΔP

Where %ΔQd is the percentage change in quantity demanded and %ΔP is the percentage change in price. The Greek letter Δ (delta) means ‘change in’.

其中 %ΔQd 表示需求量变动百分比，%ΔP 表示价格变动百分比。希腊字母 Δ 表示“变化量”。

To calculate the percentage change, use the original value as the denominator:

计算变动百分比时，以原始值作为分母：

%ΔQd = (Change in quantity ÷ Original quantity) × 100

%ΔP = (Change in price ÷ Original price) × 100

Because price and quantity move in opposite directions along the demand curve, PED is normally negative. In A-Level exams, economists often ignore the negative sign and refer to the absolute value when discussing ‘elastic’ or ‘inelastic’.

由于价格与需求量在需求曲线上呈反向变动，因此需求价格弹性通常为负值。在 A-Level 考试中，经济学者讨论“有弹性”或“无弹性”时，通常忽略负号并取绝对值。

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3. Calculating PED: Worked Example | 计算需求价格弹性：例题

Consider a firm that sells coffee. The price rises from $4 to $5 per cup, and the quantity demanded falls from 200 cups to 160 cups per day. Calculate PED.

假设一家咖啡店出售咖啡，价格从每杯 4 美元上升到 5 美元，日需求量从 200 杯下降到 160 杯。试计算需求价格弹性。

Step 1: Calculate %ΔQd

第一步：计算需求量变动百分比

%ΔQd = (160 − 200) ÷ 200 × 100 = −20%

Step 2: Calculate %ΔP

第二步：计算价格变动百分比

%ΔP = (5 − 4) ÷ 4 × 100 = +25%

Step 3: Divide %ΔQd by %ΔP

第三步：用需求量变动百分比除以价格变动百分比

PED = −20% ÷ +25% = −0.8

The absolute value is 0.8, which is less than 1, so demand is price inelastic. A 25% price rise leads to only a 20% fall in quantity demanded.

绝对值等于 0.8，小于 1，因此需求缺乏价格弹性。价格上升 25% 仅导致需求量下降 20%。

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4. Interpreting the PED Value | 解读需求价格弹性数值

The numerical value of PED determines whether demand is elastic, inelastic, or unit elastic. It is helpful to classify PED as follows:

需求价格弹性的数值决定了需求是有弹性、缺乏弹性还是单位弹性。可将 PED 分类如下：

Term | 术语	Absolute Value | 绝对值	Meaning | 含义
Elastic | 有弹性	PED > 1	Quantity demanded changes by a larger percentage than price. | 需求量变动百分比大于价格变动百分比。
Inelastic | 无弹性	PED < 1	Quantity demanded changes by a smaller percentage than price. | 需求量变动百分比小于价格变动百分比。
Unit elastic | 单位弹性	PED = 1	Quantity demanded changes by exactly the same percentage as price. | 需求量变动百分比与价格变动百分比相同。

For example, fresh fruit often has a PED greater than 1 because many close substitutes exist. Petrol in the short run has a PED less than 1 because consumers cannot easily change their driving habits quickly.

例如，新鲜水果的需求价格弹性通常大于 1，因为存在许多相近的替代品；而在短期内，汽油的需求价格弹性小于 1，因为消费者无法迅速改变驾驶习惯。

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5. The Two Extreme Cases | 两种极端情形

There are two extreme cases that A-Level students must know: perfectly inelastic demand and perfectly elastic demand.

A-Level 学生必须掌握两种极端情形：完全无弹性需求与完全有弹性需求。

Perfectly inelastic demand (PED = 0) occurs when a change in price has no effect on quantity demanded. The demand curve is vertical. A common example is a life-saving medicine that patients must buy regardless of cost.

完全无弹性需求（PED = 0）指价格的任何变动都不会引起需求量变化，需求曲线呈垂直状态。常见的例子是患者必须购买、不论价格高低都要购买的救命药物。

Perfectly elastic demand (PED = ∞) occurs when consumers will only buy at one price; any price increase causes quantity demanded to fall to zero. The demand curve is horizontal. This is typical of a perfectly competitive market where many identical products are sold at the market price.

完全有弹性需求（PED = ∞）指消费者只愿意在一个价格水平购买；价格只要稍有上升，需求量就会降为零。需求曲线呈水平状态。这常见于完全竞争市场中出售大量同质产品的情况。

In practice, these extreme cases are rare, but they are useful boundary concepts for understanding the range of PED values.

实践中，这两种极端情况很少见，但它们有助于理解需求价格弹性取值范围的边界概念。

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6. Determinants of Price Elasticity of Demand | 影响需求价格弹性的因素

Several factors determine whether a good has elastic or inelastic demand. Examiners often ask students to explain these determinants with examples.

多种因素决定一种商品的需求是有弹性还是无弹性。考试常要求学生结合例子解释这些影响因素。

• Availability of substitutes | 替代品的可获得性：

  Goods with many close substitutes have more elastic demand because consumers can easily switch if the price rises. For instance, a specific brand of cereal has elastic demand, while salt has very few substitutes and is therefore inelastic.

  拥有大量相近替代品的商品，需求更有弹性，因为消费者在价格上涨时容易转购替代品。例如，某一特定品牌的麦片需求有弹性，而食盐几乎没有替代品，因此需求无弹性。

• Necessity versus luxury | 必需品与奢侈品：

  Necessities such as bread and basic clothing have inelastic demand, while luxury goods such as designer handbags have elastic demand. Consumers can postpone or avoid luxury purchases when prices rise.

  面包、基本衣物等必需品需求缺乏弹性，而名牌手袋等奢侈品需求有弹性。价格上涨时，消费者可以推迟或取消奢侈品的消费。

• Proportion of income spent | 支出占收入的比例：

  Goods that take up a small share of income, such as chewing gum, have inelastic demand. Goods that take a large share, such as cars or housing, tend to have more elastic demand because consumers are more sensitive to price changes.

  口香糖等占收入比重很小的商品需求缺乏弹性；而汽车、住房等占收入比重大的商品需求往往更有弹性，因为消费者对价格变动更敏感。

• Time period | 时间长短：

  Demand is usually more elastic in the long run than in the short run. After a fuel price rise, consumers may initially keep driving the same amount, but over time they may buy more fuel-efficient cars or use public transport.

  长期需求通常比短期需求更有弹性。燃油价格上涨后，消费者短期内可能维持原有驾驶量，但长期可能购买更省油的汽车或改乘公共交通。

• Habit and brand loyalty | 消费习惯与品牌忠诚度：

  Goods that are strongly associated with habit or brand loyalty, such as cigarettes or a popular smartphone, tend to have inelastic demand. Loyal consumers are less willing to switch away even if the price increases.

  香烟或热门智能手机等与消费习惯或品牌忠诚度联系紧密的商品，需求往往缺乏弹性。忠诚消费者即使面对涨价也不愿转向其他品牌。

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7. PED and Total Revenue | 需求价格弹性与总收益

Total revenue is the amount a firm receives from selling its product, calculated as price × quantity. The relationship between PED and total revenue is essential for business pricing decisions.

总收益是企业销售产品所得的全部收入，计算公式为价格 × 数量。需求价格弹性与总收益的关系对企业定价决策至关重要。

PED | 弹性	Price cut | 降价	Price rise | 涨价
Elastic (PED > 1) | 有弹性	Total revenue increases | 总收益增加	Total revenue decreases | 总收益减少
Inelastic (PED < 1) | 无弹性	Total revenue falls | 总收益下降	Total revenue rises | 总收益上升
Unit elastic (PED = 1) | 单位弹性	Total revenue unchanged | 总收益不变	Total revenue unchanged | 总收益不变

For example, a bus company facing elastic demand may lower fares to attract more passengers. Because demand rises proportionally more than the fare cut, total revenue increases. Conversely, a company selling a necessity with inelastic demand may raise prices and achieve higher revenue, as the quantity lost is small.

例如，如果公交公司面对有弹性的需求，降低票价可吸引更多乘客。由于需求量上升幅度大于票价下降幅度，总收益增加。反之，如果企业销售的是缺乏弹性的必需品，涨价会带来更高的总收益，因为需求量减少的幅度很小。

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8. Applications of PED | 需求价格弹性的应用

PED is not just a theoretical concept; it has direct applications in business strategy and government policy.

需求价格弹性不只是理论概念，还直接应用于商业策略和政府政策。

Business pricing strategy: Firms use PED to decide whether to raise or lower prices. A firm with a loyal customer base and inelastic demand can increase prices to boost revenue. A firm in a competitive market with elastic demand may use discounts and promotions to increase sales volume.

企业定价策略：企业利用需求价格弹性决定涨价还是降价。拥有忠诚客户且需求缺乏弹性的企业可以提高价格以增加收益；处于竞争激烈市场、需求有弹性的企业，则可能通过折扣和促销来扩大销量。

Government taxation: When the government imposes an indirect tax on a good, the burden is passed on to consumers through higher prices. If demand is inelastic, consumers pay most of the tax. For example, cigarettes are heavily taxed because their demand is inelastic, so the government raises significant tax revenue while the fall in quantity demanded is limited.

政府税收：政府对商品征收间接税时，税负会通过涨价转嫁给消费者。如果需求缺乏弹性，消费者承担大部分税负。例如，香烟税负较重，因为香烟需求缺乏弹性，政府既能获得大量税收收入，需求量减少又较为有限。

Subsidy policy: If the government subsidises a necessary good with inelastic demand, such as basic food or public transport, the benefit mainly reduces prices for consumers rather than dramatically increasing quantity consumed.

补贴政策：如果政府对需求缺乏弹性的必需品（如基本食品或公共交通）提供补贴，补贴的收益主要体现在降低消费者价格上，而不会大幅增加消费量。

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9. Common Exam Mistakes | 常见考试误区

Students often lose marks in PED questions because of avoidable errors. Here are the most common pitfalls and how to avoid them.

学生在需求价格弹性题目中常因一些可避免的错误而失分。下面总结最常见的误区及应对方法。

• Confusing slope with elasticity: A steep demand curve is not always inelastic, and a flat curve is not always elastic. Elasticity is based on percentage changes and can vary at different points on the same straight-line demand curve.

混淆斜率与弹性：陡峭的需求曲线未必无弹性，平缓的需求曲线未必有弹性。弹性基于百分比变动，在同一条线性需求曲线的不同点上可以发生变化。

• Ignoring the minus sign: PED is normally negative. Some exam questions specifically ask for the absolute value, while others expect the negative value. Always read the question carefully and show the sign if required.

忽略负号：需求价格弹性通常为负值。有些题目要求写绝对值，有些则要求写带符号的数值。务必仔细审题并在必要时写出负号。

• Using the wrong denominator for percentage change: In CIE A-Level, unless the question asks for the midpoint method, use the original value as the denominator. Mixing up new and original values will give the wrong answer.

百分比变化的分母用错：在 CIE A-Level 中，除非题目要求使用中点法，否则应使用原始值作为分母。混淆新旧数值会导致答案错误。

• Confusing PED with cross-price or income elasticity: PED measures the own-price response of quantity demanded. Do not mix it up with cross-price elasticity of demand or income elasticity of demand.

混淆需求价格弹性与交叉弹性或收入弹性：需求价格弹性衡量的是自身价格变动对需求量的影响，不要与交叉需求弹性或收入需求弹性混淆。

• Writing ‘closer to zero’ or ‘closer to one’ without identifying elastic or inelastic: Always state clearly whether demand is elastic, inelastic, or unit elastic, and justify your answer using the PED value.

只写“靠近零”或“靠近一”，而不判断弹性类型：应明确说明需求是有弹性、无弹性还是单位弹性，并用 PED 数值说明理由。

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10. Summary and Practice Tips | 总结与练习建议

Price elasticity of demand is a vital toolkit for economic analysis. You must remember the formula, interpret the absolute value correctly, and link PED to revenue and policy outcomes.

需求的价格弹性是经济分析的重要工具。你需要牢记公式，正确解读绝对值，并将需求价格弹性与收益和政策结果联系起来。

To do well in the exam, practise drawing diagrams of elastic and inelastic demand curves, and practise calculating PED from numerical data. In essay questions, always use real-world examples to support your explanation, such as fuel, cigarettes, or branded clothing.

要在考试中取得好成绩，应练习绘制有弹性和无弹性的需求曲线图，并通过数值数据计算需求价格弹性。在论文题中，务必结合实际案例（如燃油、香烟或品牌服装）来支持你的论述。

Finally, remember a short revision checklist:

最后，请记住一个简短的复习清单：

Checklist | 清单	Question | 问题
Formula | 公式	Can I state and apply PED = %ΔQd ÷ %ΔP? | 我能否写出并运用 PED = %ΔQd ÷ %ΔP？
Interpretation | 解读	Do I know the difference between elastic, inelastic, and unit elastic? | 我能否区分有弹性、无弹性和单位弹性？
Determinants | 影响因素	Can I list at least four determinants of PED? | 我能否至少列举四个影响需求价格弹性的因素？
Total revenue | 总收益	Can I explain how price changes affect total revenue in each PED case? | 我能否解释不同弹性情形下价格变动如何影响总收益？

With regular practice and careful attention to signs and definitions, you will find PED questions straightforward and score full marks with confidence.

通过定期练习并关注符号和定义的细节，你会发现需求价格弹性题目非常直观，并且能够自信地获得满分。

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Published by TutorHao | Economics Revision Series | aleveler.com

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