PED: Price Elasticity of Demand — AS Edexcel 经济学需求价格弹性完全指南

一、什么是需求价格弹性：定义与直觉 | What Is Price Elasticity of Demand: Definition and Intuition

需求价格弹性（Price Elasticity of Demand，简称 PED）是经济学中最核心的概念之一。它衡量的是：当商品的价格发生变化时，消费者对该商品的需求量会做出多大程度的反应。简单来说，PED 回答的问题是”价格涨了，人们真的会少买吗？少买多少？”

Price Elasticity of Demand (PED) is one of the most important concepts in economics. It measures how strongly the quantity demanded of a good responds when its price changes. In simple terms, PED answers the question: “If the price goes up, will people really buy less? And how much less?”

这个反应程度并不是凭空想象的，而是可以用一个精确的数值来量化。如果价格小幅上涨导致需求量大幅下降，我们就说这种商品的需求是”富有弹性”的（elastic）；如果价格大幅上涨而需求量几乎不变，我们就说这种商品的需求是”缺乏弹性”的（inelastic）。理解这个区分，是掌握后续所有弹性分析的基础。

This responsiveness is not a matter of guesswork; it can be quantified with a precise number. If a small price rise causes a large fall in quantity demanded, we call the demand for that good “elastic”. If a large price rise leaves quantity demanded almost unchanged, we call the demand “inelastic”. Understanding this distinction is the foundation for all elasticity analysis that follows.

学习 PED 时，很多同学会犯一个直觉上的错误：认为”弹性”就是”需求曲线是不是陡峭”。实际上，弹性是一个比率概念，它与曲线的斜率相关但绝不相等。我们会在后面的章节中详细解释两者的区别。现在，请先把 PED 理解为一个纯粹的数值：它没有单位，只表示需求量变化的百分比与价格变化的百分比之间的比值。

When studying PED, many students make an intuitive mistake: they think “elasticity” simply means “whether the demand curve is steep”. In fact, elasticity is a ratio concept; it is related to the slope of the curve but by no means equal to it. We will explain the difference in detail in a later section. For now, treat PED as a pure number: it has no units and simply represents the ratio between the percentage change in quantity demanded and the percentage change in price.

二、PED 的计算公式：百分比变化法与中点法 | The PED Formula: Percentage Change and Midpoint Methods

PED 的标准计算公式是：PED = 需求量变化的百分比 ÷ 价格变化的百分比。在考试中，你通常需要先分别计算两个百分比，再相除得到最终数值。计算百分比变化的标准方法是”变化量 ÷ 原始值 × 100%”。

The standard formula for PED is: PED = percentage change in quantity demanded divided by percentage change in price. In exams, you usually calculate the two percentages separately and then divide them to obtain the final value. The standard way to calculate a percentage change is: change divided by the original value, multiplied by 100%.

我们来看一个具体的例子。假设某商品的价格从 10 元上涨到 12 元，需求量从 100 件下降到 80 件。价格的百分比变化 = (12 – 10) ÷ 10 × 100% = +20%。需求量的百分比变化 = (80 – 100) ÷ 100 × 100% = -20%。因此 PED = -20% ÷ +20% = -1。由于需求定律决定了价格与需求量总是反方向变动，PED 在数学上几乎总是负数。经济学中我们习惯取绝对值来讨论，即 |PED| = 1。

Let us look at a concrete example. Suppose the price of a good rises from 10 yuan to 12 yuan, and the quantity demanded falls from 100 units to 80 units. The percentage change in price = (12 – 10) / 10 x 100% = +20%. The percentage change in quantity demanded = (80 – 100) / 100 x 100% = -20%. Therefore PED = -20% / +20% = -1. Because the law of demand ensures that price and quantity demanded always move in opposite directions, PED is almost always negative in mathematics. In economics we habitually discuss its absolute value, so |PED| = 1.

这里有一个重要的考试陷阱：当价格下降或需求量下降时，”变化量 ÷ 原始值”中的原始值应该用变动之前的数值。但如果题目给出的是两个时间点的数据，用哪一端做分母会导致结果不同。为了避免这种歧义，许多教科书和考试局推荐使用”中点法”（midpoint method）：分母取两个数值的平均值。例如价格从 10 变到 12，中点法的分母是 (10 + 12) ÷ 2 = 11，百分比变化 = 2 ÷ 11 × 100% ≈ 18.2%。这样无论从哪个方向计算，结果都一致。

Here is an important exam trap: when price falls or quantity demanded falls, the “original value” in “change divided by original value” should be the value before the change. But when the question gives data for two points in time, using either end as the denominator can give different results. To avoid this ambiguity, many textbooks and exam boards recommend the “midpoint method”: the denominator is the average of the two values. For example, if price changes from 10 to 12, the midpoint denominator is (10 + 12) / 2 = 11, and the percentage change is 2 / 11 x 100% ≈ 18.2%. This way the result is the same no matter which direction you calculate from.

项目 Item	标准法 Standard method	中点法 Midpoint method
价格 10 到 12 的百分比变化 Price change 10 to 12	(2/10) x 100% = 20%	(2/11) x 100% ≈ 18.2%
价格 12 到 10 的百分比变化 Price change 12 to 10	(-2/12) x 100% ≈ -16.7%	(-2/11) x 100% ≈ -18.2%
两个方向是否一致 Consistent both ways?	否，结果不同 No, results differ	是，结果一致 Yes, results match

在 Edexcel AS 经济学的考试中，计算题通常直接给出价格和需求量的前后数值，标准法（用变动前数值作分母）是默认要求。但如果你在论文题中需要描述”两个时点之间的弹性”，中点法是更严谨的选择。建议同学们两种方法都熟练掌握，并在答卷中注明自己使用了哪种方法。

In Edexcel AS Economics exams, calculation questions usually give the before and after values of price and quantity; the standard method (using the pre-change value as the denominator) is the default requirement. But if you need to describe the elasticity “between two points in time” in an essay question, the midpoint method is the more rigorous choice. It is recommended that you master both methods and state clearly in your answer which one you used.

三、PED 的数值范围：弹性、单位弹性与非弹性的界限 | PED Values: Elastic, Unit Elastic and Inelastic Ranges

PED 的绝对值落在不同的区间，代表完全不同的市场行为。理解这些区间及其经济含义，是 AS 考试中高频选择题和简答题的考点。我们按 |PED| 从大到小把需求分为五大类。

The absolute value of PED falls into different ranges that represent very different market behaviours. Understanding these ranges and their economic meanings is a high-frequency topic in AS multiple-choice and short-answer questions. We divide demand into five categories according to |PED|, from largest to smallest.

第一类是”完全弹性”（perfectly elastic），此时 |PED| 趋于无穷大。价格只要上涨一点点，需求量就骤降到零，消费者会立刻转向其他卖家的完全相同产品。这通常出现在完全竞争市场中，单个企业面对的需求曲线是一条水平线。第二类是”富有弹性”（elastic），此时 |PED| > 1，需求量变化的百分比大于价格变化的百分比。例如某品牌的运动鞋，价格涨 10%，销量可能下降 20%。

The first category is “perfectly elastic”, where |PED| tends to infinity. If the price rises even a tiny amount, quantity demanded collapses to zero as consumers instantly switch to identical products from other sellers. This typically appears in perfectly competitive markets, where the demand curve facing a single firm is a horizontal line. The second category is “elastic”, where |PED| > 1 and the percentage change in quantity demanded exceeds the percentage change in price. For example, a sports shoe brand might see sales fall 20% when its price rises 10%.

第三类是”单位弹性”（unit elastic），此时 |PED| = 1，需求量与价格按相同百分比变动。这是一种理论上的分界点，实际商品很少恰好落在这一数值上，但它对于理解总收入的变化至关重要（见第六节）。第四类是”缺乏弹性”（inelastic），此时 0 < |PED| < 1，需求量变化的百分比小于价格变化的百分比。食盐、汽油、香烟都是典型例子：价格上涨，人们只能减少少量消费。第五类是”完全无弹性”（perfectly inelastic），此时 |PED| = 0，价格无论如何变化，需求量都保持不变。胰岛素等救命的必需品最接近这一情况。

The third category is “unit elastic”, where |PED| = 1 and quantity demanded changes by the same percentage as price. This is a theoretical dividing point; few real goods sit exactly on it, but it is crucial for understanding changes in total revenue (see Section 6). The fourth category is “inelastic”, where 0 < |PED| < 1 and the percentage change in quantity demanded is smaller than the percentage change in price. Salt, petrol and cigarettes are typical examples: when the price rises, people can only cut consumption slightly. The fifth category is “perfectly inelastic”, where |PED| = 0 and quantity demanded stays the same no matter how price changes. Life-saving necessities such as insulin come closest to this case.

类型 Type	|PED| 取值 Value	含义 Meaning	典型例子 Example
完全弹性 Perfectly elastic	无穷大 Infinity	价格微涨即失去全部需求 Tiny price rise loses all demand	完全竞争企业的产品 A firm in perfect competition
富有弹性 Elastic	> 1	需求量变化更剧烈 Quantity changes more than price	名牌服装、奢侈品 Designer clothing, luxury goods
单位弹性 Unit elastic	= 1	两者同比例变动 Both change by same percentage	理论分界点 Theoretical boundary
缺乏弹性 Inelastic	0 到 1 Between 0 and 1	需求量变化较温和 Quantity changes less than price	汽油、香烟、食盐 Petrol, cigarettes, salt
完全无弹性 Perfectly inelastic	= 0	需求量完全不变 Quantity unchanged	胰岛素等救命药 Life-saving medicine

考试中常考的另一种表述方式是”需求曲线越陡峭，弹性越小”。这个说法在大多数情况下成立，但严格来说只对通过同一点的两条线性需求曲线成立。我们会在第五节用图形语言解释这个细微差别。

Another common exam statement is “the steeper the demand curve, the smaller the elasticity”. This is true in most cases, but strictly speaking it only holds for two linear demand curves passing through the same point. We will use graphical language to explain this subtlety in Section 5.

四、完全弹性与完全无弹性：两种极端情况的图形与意义 | Perfectly Elastic and Perfectly Inelastic Demand: Graphs and Significance

完全弹性和完全无弹性是两条极端的需求曲线，它们在图形上分别是水平线和垂直线。完全弹性需求曲线是一条水平直线：在市场价格 P 上，企业可以卖出任意数量；但只要价格高于 P，需求量立即降为零。现实中最接近的例子是小麦等标准化农产品，单个农户面对的是近乎水平的需求曲线，因为他的产品与别人的完全同质，买家可以随意换一个卖主。

Perfectly elastic and perfectly inelastic demand are two extreme demand curves; graphically they are a horizontal line and a vertical line respectively. The perfectly elastic demand curve is a horizontal straight line: at the market price P, the firm can sell any quantity; but if the price rises above P, quantity demanded immediately drops to zero. The closest real-world example is a standardised agricultural product such as wheat. A single farmer faces a nearly horizontal demand curve because his product is completely homogeneous with everyone else’s and buyers can freely switch to another seller.

完全无弹性需求曲线是一条垂直线：无论价格如何变化，需求量都固定在 Q。这通常意味着该商品没有任何替代品，而且消费者必须购买它才能生存。现实中的例子包括某些专利药、急救服务等。需要注意的是，完全无弹性是一个极端假设，现实中即使胰岛素，当价格高到一定程度时，部分患者也可能减少用量或寻找替代治疗方案，因此真实世界的需求几乎总是带有一定的弹性。

The perfectly inelastic demand curve is a vertical line: no matter how price changes, quantity demanded stays fixed at Q. This usually means the good has no substitutes and consumers must buy it to survive. Real examples include certain patented drugs and emergency medical services. Note that perfect inelasticity is an extreme assumption; in reality, even for insulin, when the price rises high enough some patients may reduce dosages or seek alternative treatments, so real-world demand almost always has some elasticity.

从政府政策的角度看，这两种极端情况有非常重要的含义。对于完全无弹性的商品（如某些药品），政府征收间接税时，税收负担几乎全部由消费者承担，因为消费者没有其他选择，无法通过减少购买来”逃税”。而对于完全弹性的商品，企业无法把任何税负转嫁给消费者，因为一旦提价就会失去全部销量。这个逻辑我们在第七节会进一步展开。

From the perspective of government policy, these two extremes have very important implications. For perfectly inelastic goods (such as certain medicines), when the government imposes an indirect tax, almost the entire tax burden falls on consumers, because consumers have no alternative and cannot “escape” the tax by buying less. For perfectly elastic goods, firms cannot pass any of the tax burden onto consumers, because raising the price would lose them all their sales. We will develop this logic further in Section 7.

五、决定 PED 的五大因素：替代品、必需品、时间、预算占比与习惯 | Five Determinants of PED: Substitutes, Necessities, Time, Budget Share and Habit

为什么有的商品弹性大，有的商品弹性小？AS 考试要求你能够列出并解释决定 PED 的因素。掌握这五个因素，不仅是简答题的得分点，也是论述题中分析现实市场的基础。以下按照考试出题频率排序。

Why are some goods elastic and others inelastic? AS exams require you to list and explain the factors that determine PED. Mastering these five factors is not only a scoring point in short-answer questions but also the foundation for analysing real markets in essays. They are listed below in order of exam frequency.

第一个因素是替代品的可得性（availability of substitutes）。这是最重要的决定因素：替代品越多、越相近，消费者在涨价时就越容易转向其他选择，弹性就越大。例如，某个牌子的矿泉水有很多相近的替代品，其需求弹性很大；而食盐几乎没有替代品，弹性很小。第二个因素是商品是否为必需品（necessity）还是奢侈品（luxury）。必需品如食物、水电，消费者无法轻易放弃，弹性小；奢侈品如度假、高档餐厅，弹性大。

The first factor is the availability of substitutes. This is the most important determinant: the more and the closer the substitutes, the easier it is for consumers to switch when the price rises, and the larger the elasticity. For example, one brand of bottled water has many close substitutes, so its demand is highly elastic; salt has almost no substitutes, so its elasticity is tiny. The second factor is whether the good is a necessity or a luxury. Necessities such as food and utilities cannot be easily given up, so their elasticity is small; luxuries such as holidays and fine dining have large elasticity.

第三个因素是时间的长短（time period）。价格变动后的时间越短，消费者越难调整消费习惯和寻找替代品，弹性越小；时间越长，消费者越容易找到替代方案（例如换用更省油的车型、加装隔热层），弹性越大。这也是为什么短期油价上涨对需求影响很小，而长期需求会逐渐调整。第四个因素是商品支出占消费者预算的比例（proportion of income spent on the good）。占比越大，消费者对价格越敏感，弹性越大。例如，住房支出占收入比例高，价格变化对需求的影响就明显；而火柴、盐这类小开支商品，价格翻倍也不会显著改变需求。

The third factor is the time period. The shorter the time after a price change, the harder it is for consumers to adjust their habits and find substitutes, so elasticity is small; the longer the time, the easier it is to find alternatives (for example, switching to a more fuel-efficient car or adding insulation), so elasticity grows. This is why short-run petrol price rises have little effect on demand, while demand adjusts gradually in the long run. The fourth factor is the proportion of income spent on the good. The larger the share, the more price-sensitive consumers are, and the larger the elasticity. Housing, for example, absorbs a large share of income, so price changes visibly affect demand; for small-ticket items like matches or salt, even a doubling of price hardly changes demand.

第五个因素是习惯与成瘾性（habit and addiction）。香烟、酒精、赌博类商品即使大幅涨价，忠实消费者也较难戒断，因此需求缺乏弹性。此外，商品的”宽度定义”也会影响弹性：定义越宽，替代品越少，弹性越小。例如”食物”整体几乎没有替代品，弹性很小；而”苹果”有香蕉、橙子等大量替代品，弹性很大。这个”窄定义更富弹性”的原理，是选择题的常见考点。

The fifth factor is habit and addiction. For goods such as cigarettes, alcohol and gambling, even large price rises are hard for loyal consumers to give up, so demand is inelastic. In addition, the “breadth of definition” of a good also matters: the broader the definition, the fewer the substitutes, and the smaller the elasticity. For example, “food” as a whole has almost no substitutes and is very inelastic, while “apples” have many substitutes such as bananas and oranges and are very elastic. The principle that “narrower definitions are more elastic” is a common multiple-choice question.

六、需求曲线斜率与 PED：为什么”陡峭”并不总是等于”缺乏弹性” | Demand Curve Slope vs PED: Why “Steeper” Is Not Always “Less Elastic”

很多同学看到一条陡峭的需求曲线就断定它缺乏弹性，看到平缓的曲线就说它富有弹性。这个直觉在比较通过同一点的两条曲线时是正确的，但在一般情形下却是一个危险的误解。关键在于：弹性是百分比之比，而斜率是绝对量之比。

Many students look at a steep demand curve and conclude it is inelastic, and a flat curve and call it elastic. This intuition is correct when comparing two curves through the same point, but in general it is a dangerous misconception. The key point is that elasticity is a ratio of percentages, while slope is a ratio of absolute quantities.

我们以一条线性需求曲线 Q = 100 – P 为例。它的斜率恒为 -1，但弹性却在每个点上都不同。在价格很高、需求量很小的左上端（例如 P = 90, Q = 10），需求量的小幅绝对变化对应巨大的百分比变化，因此 |PED| 很大；在价格很低、需求量很大的右下端（例如 P = 10, Q = 90），同样幅度的绝对变化只占很小的百分比，因此 |PED| 很小。事实上，任何线性需求曲线的中点处 |PED| 恰好等于 1：中点以上富有弹性，中点以下缺乏弹性。

Take a linear demand curve Q = 100 – P as an example. Its slope is always -1, but its elasticity is different at every point. At the top-left end where price is high and quantity is small (say P = 90, Q = 10), a small absolute change in quantity represents a huge percentage change, so |PED| is large; at the bottom-right end where price is low and quantity is large (say P = 10, Q = 90), the same absolute change represents only a tiny percentage, so |PED| is small. In fact, at the midpoint of any linear demand curve |PED| equals exactly 1: above the midpoint demand is elastic, below it demand is inelastic.

这个”线性需求曲线弹性处处不同”的结论有一个重要的推论：当企业沿着需求曲线降价时，它从弹性区间进入非弹性区间，总收入先增后减，在 |PED| = 1 的点上总收入达到最大。这就把我们带到了下一节：总收入与 PED 的关系。考试中经常用”降价是否总能增加收入”这类问题来考察这一点，答案是否定的，只有当需求富有弹性时降价才会增加总收入。

This conclusion that “elasticity differs at every point on a linear demand curve” has an important corollary: when a firm lowers its price along the demand curve, it moves from the elastic range into the inelastic range, total revenue first rises then falls, and reaches its maximum exactly at the point where |PED| = 1. This leads us to the next section: the relationship between total revenue and PED. Exams often test this with questions such as “does lowering the price always increase revenue?”; the answer is no. Lowering the price increases total revenue only when demand is elastic.

七、总收入与 PED：涨价与降价的收入效应 | Total Revenue and PED: The Revenue Effect of Price Changes

总收入（Total Revenue, TR）等于价格乘以需求量（TR = P x Q）。PED 决定了价格变动后总收入向哪个方向变化，这是企业经营决策和政府定价政策中最实际的应用之一。我们先建立直观：价格上升时，每一单位产品卖出更多钱（有利），但同时卖出的数量变少（不利）。最终结果取决于哪个效应更强，而哪个效应更强，恰恰由 PED 决定。

Total revenue (TR) equals price times quantity demanded (TR = P x Q). PED determines in which direction total revenue moves after a price change; this is one of the most practical applications of the concept in business decisions and government pricing policy. Let us first build the intuition: when the price rises, each unit sold earns more money (favourable), but fewer units are sold (unfavourable). The final outcome depends on which effect is stronger, and which effect is stronger is precisely determined by PED.

当需求富有弹性（|PED| > 1）时，数量减少的百分比大于价格上升的百分比，因此涨价会使总收入下降，降价反而会使总收入上升。当需求缺乏弹性（|PED| < 1）时，数量减少的百分比小于价格上升的百分比，涨价会使总收入上升，降价会使总收入下降。当需求为单位弹性（|PED| = 1）时，两个效应正好抵消，价格变动不改变总收入。

When demand is elastic (|PED| > 1), the percentage fall in quantity exceeds the percentage rise in price, so a price rise reduces total revenue and a price cut increases it. When demand is inelastic (|PED| < 1), the percentage fall in quantity is smaller than the percentage rise in price, so a price rise increases total revenue and a price cut reduces it. When demand is unit elastic (|PED| = 1), the two effects exactly cancel out and a price change leaves total revenue unchanged.

这个关系的考试价值极高，因为它把代数、图形和经济直觉连成一条线。举个例子：某音乐会主办方把票价从 50 元提高到 60 元，上座人数从 2000 人下降到 1800 人。需求量的百分比变化 = -10%，价格的百分比变化 = +20%，所以 |PED| = 0.5，需求缺乏弹性。涨价前总收入 = 50 x 2000 = 100000 元；涨价后总收入 = 60 x 1800 = 108000 元。可见在缺乏弹性的情况下涨价确实增加了收入。许多企业误以为”降价促销总能多赚钱”，只有需求富有弹性时才成立；否则降价反而会减少总收入，这正是价格战常常两败俱伤的原因。

This relationship has extremely high exam value because it connects algebra, graphs and economic intuition in one chain. As an example: a concert organiser raises the ticket price from 50 yuan to 60 yuan, and attendance falls from 2000 to 1800. The percentage change in quantity is -10%, the percentage change in price is +20%, so |PED| = 0.5 and demand is inelastic. Before the rise, TR = 50 x 2000 = 100000 yuan; after the rise, TR = 60 x 1800 = 108000 yuan. So under inelastic demand, raising the price does indeed increase revenue. Many firms mistakenly believe that “price promotions always make more money”; this only holds when demand is elastic. Otherwise a price cut reduces revenue, which is why price wars often hurt both sides.

弹性区间 Elasticity range	涨价的效果 Effect of price rise	降价的效果 Effect of price cut
富有弹性 Elastic (> 1)	总收入下降 TR falls	总收入上升 TR rises
单位弹性 Unit elastic (= 1)	总收入不变 TR unchanged	总收入不变 TR unchanged
缺乏弹性 Inelastic (< 1)	总收入上升 TR rises	总收入下降 TR falls

考试中画图时，请务必用”弹性区间”而不是”曲线的陡峭程度”来标注总收入的变化：在需求曲线的弹性区段标注收入增加或减少的箭头，并用阴影矩形直观展示 TR 的变化。把矩形面积的变化与 PED 的数值对应起来，是高分答案的常见特征。

When drawing diagrams in exams, be sure to label changes in total revenue using “elasticity ranges” rather than “curve steepness”: mark arrows for revenue increases or decreases on the elastic section of the demand curve, and show the change in TR visually with shaded rectangles. Linking the change in rectangle area to the value of PED is a common feature of high-scoring answers.

八、PED 与间接税：税收负担如何分摊 | PED and Indirect Tax: How the Tax Burden Is Shared

政府征收间接税（如增值税、燃油税、烟草税）时，税负在消费者与生产者之间的分摊比例由供求双方的弹性共同决定。对于需求侧的 PED，规律非常清晰：需求越缺乏弹性，消费者承担的税负比例越大；需求越富有弹性，生产者承担的税负比例越大。极端情况下，完全无弹性需求下消费者承担全部税负，完全弹性需求下生产者承担全部税负。

When the government imposes an indirect tax (such as VAT, fuel duty or tobacco tax), the share of the burden between consumers and producers is jointly determined by the elasticities of supply and demand. On the demand side, the rule is very clear: the more inelastic the demand, the larger the share of the tax borne by consumers; the more elastic the demand, the larger the share borne by producers. In the extreme, consumers bear the whole burden under perfectly inelastic demand, and producers bear the whole burden under perfectly elastic demand.

我们用图形来理解。对商品征收每单位 t 元的税，会使供给曲线向上平移 t 元。新均衡价格上升，但上升的幅度小于 t（除非需求完全无弹性），因为生产者无法把全部税负转嫁出去。消费者支付的价格从 P1 上升到 P2，生产者实际获得的价格从 P1 下降到 P3，P2 与 P3 之差恰好等于税额 t。消费者多付的部分（P2 – P1）除以总税负 t，就是消费者承担的份额。

Let us understand this with a diagram. A tax of t yuan per unit shifts the supply curve up by t. The new equilibrium price rises, but by less than t (unless demand is perfectly inelastic), because producers cannot pass on the whole burden. The price consumers pay rises from P1 to P2, while the price producers actually receive falls from P1 to P3, and the gap between P2 and P3 equals the tax t. The extra amount consumers pay (P2 – P1) divided by the total tax t is the share borne by consumers.

一个常考的现实案例是香烟和燃油：它们的需求在短期内高度缺乏弹性，因此消费税的负担绝大部分落在消费者身上。政府喜欢对这类商品征税，一方面因为税收收入稳定可预测，另一方面可以发挥”矫正外部性”的作用（吸烟、排放污染的社会成本）。但同学们要注意，这类税也会带来无谓损失（deadweight loss），且对低收入群体的负担更重，即所谓的”累退性”（regressive）问题。论述题中能够同时讨论效率和公平，是拿到高分的标志。

A commonly tested real-world case is cigarettes and fuel: their demand is highly inelastic in the short run, so the burden of excise duties falls overwhelmingly on consumers. Governments like taxing such goods because tax revenue is stable and predictable, and because such taxes can correct externalities (the social costs of smoking and pollution). But note that these taxes also create deadweight loss and weigh more heavily on low-income groups, the so-called “regressive” problem. In essays, being able to discuss both efficiency and equity is a mark of a high-scoring answer.

九、PED 的考试应用：计算题与论述题的答题框架 | PED in Exams: Frameworks for Calculation and Essay Questions

在 Edexcel AS 经济学的试卷中，PED 出现在选择题、计算题、数据分析题和论述题中。每一种题型都有对应的答题套路，掌握这些套路可以显著提高得分效率。我们先看计算题的规范步骤。

In Edexcel AS Economics papers, PED appears in multiple-choice questions, calculations, data-response questions and essays. Each question type has its own answering routine; mastering these routines significantly improves scoring efficiency. Let us first look at the standard steps for calculations.

计算题的满分四步是：第一步，写出公式 PED = %变化(Qd) ÷ %变化(P)；第二步，分别计算两个百分比变化并保留到小数点后一位；第三步，相除得到 PED 并取绝对值；第四步，用文字判断弹性类型（富有弹性/缺乏弹性/单位弹性）并解释其经济含义。不要只写数字不写结论，判卷标准中”解释”通常占一半分值。另外注意单位：PED 没有单位，不要在答案后写”元”或”件”。

The four-step routine for a full-mark calculation is: first, write the formula PED = %change(Qd) / %change(P); second, calculate the two percentage changes and round to one decimal place; third, divide to obtain PED and take the absolute value; fourth, state in words the type of elasticity (elastic / inelastic / unit elastic) and explain its economic meaning. Do not write only numbers without a conclusion; in the mark scheme, “explanation” usually earns half the marks. Also watch the units: PED has no units, so never write “yuan” or “units” after your answer.

论述题（例如”分析政府提高燃油税对消费者支出的影响”）建议采用四段式框架：第一段定义并计算或估计 PED；第二段用决定因素解释该商品为何具有这样的弹性（短期缺乏弹性、长期弹性增大）；第三段结合总收入与税负分摊理论分析对不同群体的影响；第四段评价政策效果并给出其他考虑（公平性、外部性、替代政策）。每段都要有图、有数据、有经济学术语，避免纯文字描述。

For essay questions (for example, “analyse the effect of a higher fuel duty on consumer spending”), a four-paragraph framework is recommended: the first paragraph defines and calculates or estimates the PED; the second uses the determinants to explain why the good has such elasticity (inelastic in the short run, more elastic in the long run); the third combines total revenue and tax-burden theory to analyse the impact on different groups; the fourth evaluates the policy and considers other issues (equity, externalities, alternative policies). Every paragraph should include diagrams, data and economic terminology, avoiding pure prose description.

常见失分点还包括：混淆 PED 与收入弹性（YED）；把”需求量变化”写成”需求变化”（前者是沿曲线移动，后者是曲线平移）；忘记取绝对值；以及在解释弹性时只背定义不联系实际数据。建议同学们在复习时准备一张”PED 考点自查表”，把每个考点对应的图表和例题写上去，考前反复过目。

Common mark-losing points include: confusing PED with income elasticity of demand (YED); writing “change in demand” when you mean “change in quantity demanded” (the former is a shift of the curve, the latter a movement along it); forgetting to take the absolute value; and reciting definitions without linking them to actual data when explaining elasticity. It is recommended that you prepare a “PED checklist” during revision, writing the diagrams and example questions for each point on it and reviewing it repeatedly before the exam.

十、PED 的现实案例：燃油、香烟、奢侈品与公共交通 | Real-World Cases of PED: Fuel, Cigarettes, Luxury Goods and Public Transport

把理论与现实联系起来，是 AS 经济学论述题拿高分的关键能力。我们选取四个经典案例，分析它们的弹性特征和政策含义，这些素材可以直接用在考试答案中。

Connecting theory to reality is the key skill for scoring highly in AS Economics essays. We select four classic cases, analyse their elasticity characteristics and policy implications, and these materials can be used directly in exam answers.

第一个案例是燃油。短期来看，燃油需求高度缺乏弹性（|PED| 约 0.2 到 0.5），因为车主短期内无法改变通勤距离或更换车辆；长期来看弹性增大（约 0.6 到 1.0），因为人们可以换购节能车、选择公共交通或搬到离工作地点更近的地方。这也是为什么燃油税是政府稳定收入的重要来源，同时也是一个有争议的累退税。第二个案例是香烟：由于成瘾性，香烟的 PED 大约在 0.3 到 0.5 之间，属于缺乏弹性。政府对烟草征收高额税收既能增加收入，又能通过提高价格来抑制消费，但研究表明，对青少年等价格敏感群体，弹性相对更高，因此征税对减少青少年吸烟的效果更好。

The first case is fuel. In the short run, the demand for fuel is highly inelastic (|PED| around 0.2 to 0.5) because drivers cannot change their commuting distance or switch vehicles quickly; in the long run elasticity rises (around 0.6 to 1.0) as people buy more efficient cars, use public transport or move closer to work. This is why fuel duty is an important source of stable government revenue, but also a controversial regressive tax. The second case is cigarettes: because of addiction, the PED of cigarettes is roughly 0.3 to 0.5, which is inelastic. Governments impose high taxes on tobacco to raise revenue and to curb consumption through higher prices, but studies show that price-sensitive groups such as teenagers have relatively higher elasticity, so taxation is more effective at reducing teenage smoking.

第三个案例是奢侈品，例如高端手表、名牌包和豪华旅游。这类商品通常有大量替代品（不同品牌之间可以互相替代），且支出占收入比例高，因此需求富有弹性（|PED| > 1）。在经济衰退期，奢侈品行业往往最先受到冲击，这正是弹性大的表现；而在经济繁荣期，奢侈品企业可以通过小幅降价大幅刺激销量。第四个案例是公共交通：通勤者在短期内很难放弃地铁或公交（缺乏替代品），因此需求缺乏弹性；但长期来看，随着私家车、共享单车、远程办公等替代方案增多，公共交通需求的弹性会明显增大。政府在对公交票价定价时，必须考虑这一动态变化。

The third case is luxury goods, such as high-end watches, designer bags and premium travel. These goods usually have many substitutes (different brands can replace each other) and absorb a large share of income, so demand is elastic (|PED| > 1). During a recession, the luxury industry is often the first to be hit, which is exactly the manifestation of high elasticity; during a boom, luxury firms can stimulate sales significantly with modest price cuts. The fourth case is public transport: commuters can hardly abandon the metro or bus in the short run (few substitutes), so demand is inelastic; but in the long run, as alternatives such as private cars, shared bikes and remote working multiply, the elasticity of public transport demand increases noticeably. When pricing transit fares, governments must take this dynamic change into account.

在考场上运用这些案例时，建议遵循”案例 + 数据 + 理论”的三段式：先点出案例及其弹性数值范围，再用决定因素解释为什么，最后用 PED 理论（总收入、税负分摊）分析政策含义。这样的答案既有知识深度，又显示出经济学思维，是 A 等级答案的典型特征。

When using these cases in the exam hall, follow the “case + data + theory” pattern: first state the case and its elasticity range, then explain why using the determinants, and finally analyse the policy implications with PED theory (total revenue, tax burden sharing). Such answers show both depth of knowledge and economic thinking, and are typical of A-grade responses.

Summary | 总结

需求价格弹性（PED）衡量需求量对价格变化的敏感程度，计算公式为需求量变化的百分比除以价格变化的百分比，结果通常取绝对值。根据 |PED| 的大小，需求可分为完全弹性、富有弹性、单位弹性、缺乏弹性和完全无弹性五类，它们分别对应不同的图形、商品类型和政策含义。

Price elasticity of demand (PED) measures how sensitive quantity demanded is to a change in price. It is calculated by dividing the percentage change in quantity demanded by the percentage change in price, and the result is usually taken as an absolute value. According to the size of |PED|, demand can be classified into five categories: perfectly elastic, elastic, unit elastic, inelastic and perfectly inelastic, each corresponding to different diagrams, goods and policy implications.

决定 PED 的五个关键因素是：替代品的可得性、商品是否为必需品、时间长短、支出占收入的比例，以及习惯与成瘾性。总收入与 PED 的关系是：需求富有弹性时降价增加收入，缺乏弹性时涨价增加收入，单位弹性时收入不变。政府征税时，需求越缺乏弹性，消费者承担的税负越多。

The five key determinants of PED are: the availability of substitutes, whether the good is a necessity, the time period, the proportion of income spent, and habit and addiction. The relationship between total revenue and PED is: when demand is elastic, a price cut raises revenue; when inelastic, a price rise raises revenue; when unit elastic, revenue is unchanged. When the government imposes a tax, the more inelastic the demand, the more of the burden consumers bear.

复习时请重点掌握：PED 的计算与四步答题框架、弹性区间的图形判断、线性需求曲线上弹性处处不同的原理，以及燃油、香烟、奢侈品、公共交通四个现实案例。把这些知识点连成体系，你会发现 PED 不仅是考试重点，也是理解真实市场中企业定价和政府政策的万能钥匙。

When revising, focus on: the calculation of PED and the four-step answering framework, judging elasticity ranges from diagrams, the principle that elasticity differs at every point on a linear demand curve, and the four real-world cases of fuel, cigarettes, luxury goods and public transport. Link these points into a system, and you will find that PED is not only an exam priority but also a master key to understanding firm pricing and government policy in real markets.

更多咨询请联系16621398022（同微信）