IB Economics: Elasticity Revision Guide | IB 经济：弹性考点精讲

📚 IB Economics: Elasticity Revision Guide | IB 经济：弹性考点精讲

Elasticity is a cornerstone concept in IB Economics, measuring the responsiveness of one variable to changes in another. Understanding elasticity allows economists to predict how changes in prices, income, or related goods affect quantity demanded and supplied, and to evaluate policy outcomes such as tax burdens and subsidy benefits.

弹性是 IB 经济学的核心概念，衡量一个变量对另一个变量变化的反应程度。理解弹性有助于经济学家预测价格、收入或相关商品的变化如何影响需求量和供给量，并评估税收负担和补贴收益等政策效果。

1. Defining Elasticity | 弹性的定义

In economics, elasticity measures the percentage change in one variable resulting from a percentage change in another variable. It is a unit-free measure, allowing us to compare the sensitivity of different goods and services.

在经济学中，弹性衡量一个变量因另一变量发生百分比变化而产生的百分比变化。它是一种无量纲度量，使我们能够比较不同商品和服务的敏感程度。

Elasticity = (%ΔY) / (%ΔX)

2. Price Elasticity of Demand (PED) | 需求价格弹性

Price elasticity of demand (PED) measures how much the quantity demanded of a good responds to a change in its price. Formally, PED = percentage change in quantity demanded / percentage change in price.

需求价格弹性 (PED) 衡量一种商品的需求量对其价格变化的反应程度。公式为：PED = 需求量变化百分比 / 价格变化百分比。

PED = (%ΔQ_d) / (%ΔP)

Because of the law of demand, PED is usually negative. Economists often refer to its absolute value |PED| when interpreting elasticity.

由于需求定律，PED 通常为负值。经济学家在解释弹性时通常使用其绝对值 |PED|。

Key values: |PED| > 1 → elastic (quantity is very responsive); |PED| < 1 → inelastic (quantity is not very responsive); |PED| = 1 → unit elastic; |PED| = 0 → perfectly inelastic; |PED| → ∞ → perfectly elastic.

关键值：|PED| > 1 → 富有弹性（需求量反应强烈）；|PED| < 1 → 缺乏弹性（需求量反应不强烈）；|PED| = 1 → 单位弹性；|PED| = 0 → 完全无弹性；|PED| → ∞ → 完全弹性。

To calculate PED from two points, the midpoint (arc) formula is recommended to maintain consistency regardless of direction:
PED = [(Q₂−Q₁)/((Q₁+Q₂)/2)] / [(P₂−P₁)/((P₁+P₂)/2)]

若要根据两点计算 PED，建议使用中点（弧）公式，以确保无论方向如何结果一致：
PED = [(Q₂−Q₁)/((Q₁+Q₂)/2)] / [(P₂−P₁)/((P₁+P₂)/2)]

3. Determinants of PED | 需求价格弹性的决定因素

Substitutability: Goods with many close substitutes have more elastic demand; consumers can easily switch if the price rises.

替代性：具有许多近似替代品的商品需求更富弹性；价格上涨时消费者可轻易转向其他选择。

Necessity vs. luxury: Necessities tend to have inelastic demand, while luxuries have elastic demand.

必需品与奢侈品：必需品往往需求缺乏弹性，而奢侈品需求富有弹性。

Definition of the market: Narrowly defined markets (e.g., a specific brand) have more elastic demand than broadly defined markets (e.g., food in general).

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