📚 Price Elasticity of Demand | 需求价格弹性
Price elasticity of demand (PED) is one of the most frequently examined concepts in Cambridge International AS Level Economics. It measures how responsive the quantity demanded of a good is to a change in its own price, and it lies at the heart of microeconomic analysis. This article provides a complete, syllabus-aligned treatment of PED, including its definition, formula, types, determinants, relationship with total revenue, and real-world applications, together with common examination pitfalls to avoid.
需求价格弹性（PED）是剑桥国际AS经济学考试中最常考查的概念之一。它衡量商品需求量对其自身价格变动的反应程度，是微观经济分析的核心。本文提供一套完整的、与考纲对齐的PED解读，涵盖定义、公式、类型、决定因素、与总收益的关系以及现实应用，并指出常见的考试陷阱。
1. What Is Price Elasticity of Demand? | 什么是需求价格弹性？
Price elasticity of demand measures the percentage change in quantity demanded of a good resulting from a percentage change in its own price. It tells economists and businesses whether consumers are highly sensitive (elastic) or relatively insensitive (inelastic) to price movements along a demand curve.
需求价格弹性衡量的是：商品需求量变动的百分比，相对于其自身价格变动的百分比。它让经济学家和企业了解消费者对价格变动是高敏感（有弹性）还是相对不敏感（缺乏弹性）的。
In the Cambridge AS syllabus, PED is introduced as a quantitative tool that refines the law of demand. While the law of demand states that price and quantity demanded move in opposite directions, PED tells us the magnitude of that response — a crucial distinction for decision-making.
在剑桥AS考纲中，PED作为一种定量工具引入，用以深化对需求定律的认识。需求定律指出价格与需求量反方向变动，而PED告诉我们这一反应的幅度——这对决策至关重要。
2. The Formula and Calculation | 公式与计算
The basic formula for price elasticity of demand is:
PED = Percentage Change in Quantity Demanded ÷ Percentage Change in Price
The percentage change in a variable is calculated as: (new value − old value) ÷ old value × 100%. Suppose the price of a coffee falls from $3.00 to $2.50 and, as a result, the quantity demanded rises from 100 cups to 130 cups per day. The percentage change in price is (2.50 − 3.00) ÷ 3.00 × 100% = −16.67%. The percentage change in quantity demanded is (130 − 100) ÷ 100 × 100% = +30%. Therefore PED = 30 ÷ (−16.67) = −1.8. Since demand curves normally slope downwards, PED is usually negative, and economists often quote the absolute value.
百分比变化的计算公式为：（新值 − 旧值）÷ 旧值 × 100%。假设咖啡价格从3.00美元降至2.50美元，需求量从每日100杯增加到130杯。价格变动百分比为 (2.50 − 3.00) ÷ 3.00 × 100% = −16.67%。需求变动百分比为 (130 − 100) ÷ 100 × 100% = +30%。因此 PED = 30 ÷ (−16.67) = −1.8。由于需求曲线通常向下倾斜，PED通常为负值，经济学家常引用其绝对值。
The midpoint method is recommended in exams for accuracy when two price–quantity combinations are given. It divides by the average of the old and new values, making the elasticity value independent of whether price rises or falls.
考试中推荐使用中点法以保证准确性，尤其当题目给出两组价格—数量组合时。中点法以新旧数值的平均值作分母，使弹性值与价格变动方向无关。
PED = [(Q₂ − Q₁) ÷ ((Q₁ + Q₂) ÷ 2)] ÷ [(P₂ − P₁) ÷ ((P₁ + P₂) ÷ 2)]
3. Types of Price Elasticity | 价格弹性的类型
Economists classify PED into five broad categories, each with a distinct numerical range and graphical interpretation.
经济学家将PED分为五大类别，每一类都有独特的数值范围和图形含义。
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Perfectly inelastic demand (PED = 0): Quantity demanded does not change at all when price changes. The demand curve is vertical. Examples include life-saving drugs with no close substitute, such as insulin for a diabetic patient. Even if the price rises substantially, the patient must still buy the same quantity.
完全无弹性需求（PED = 0）：需求量完全不受价格变动影响。需求曲线为垂直直线。例如没有替代品的救命药物——糖尿病患者的胰岛素。即使价格大幅上涨，患者仍须购买同等数量。
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Inelastic demand (0 < PED < 1): Quantity demanded changes by a smaller percentage than price. Goods with inelastic demand include petrol, salt, and cigarettes. A 10% price rise might cause only a 3% fall in quantity demanded, so PED = 0.3.
缺乏弹性需求（0 < PED < 1）：需求量变动百分比小于价格变动百分比。汽油、食盐、香烟等商品均属此类。价格上涨10%可能仅导致需求量下降3%，即 PED = 0.3。
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Unit elastic demand (PED = 1): Quantity demanded changes by exactly the same
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