📚 AS WJEC Economics: Quick Reference Handbook of Formulae and Theorems | AS WJEC 经济：公式定理速查手册
This handbook compiles the essential formulae and economic theorems required for the AS WJEC Economics specification. Use it to fast-check key definitions, calculations, and relationships when tackling data-response and essay questions.
本手册汇编了 AS WJEC 经济学考试所需的核心公式与经济定理。你可以利用它快速查阅关键定义、计算方法和变量关系，从而更从容地应对数据分析和论文题目。
1. Price Elasticity of Demand (PED) | 需求的价格弹性
Price elasticity of demand (PED) measures the responsiveness of quantity demanded to a change in the price of the good itself.
需求的价格弹性 (PED) 衡量的是商品自身价格变化所引发的需求量反应程度。
The formula is the percentage change in quantity demanded divided by the percentage change in price. It is conventionally reported as an absolute value.
公式为需求量变动百分比除以价格变动百分比。通常取其绝对值进行报告。
PED = (%ΔQd) / (%ΔP)
If the absolute value of PED is greater than 1, demand is price elastic; a price fall raises total revenue. If it is less than 1, demand is price inelastic; a price rise raises total revenue. When PED equals 1, demand is unit elastic and total revenue remains constant.
若 PED 绝对值大于 1，则需求富有价格弹性，降价会增加总收益。若小于 1，需求缺乏价格弹性，涨价会增加总收益。当 PED 等于 1 时，需求为单位弹性，总收益保持不变。
2. Income Elasticity of Demand (YED) | 需求的收入弹性
Income elasticity of demand (YED) indicates how quantity demanded responds to a change in consumers' real income.
需求的收入弹性 (YED) 反映消费者实际收入变化对需求量的影响程度。
YED = (%ΔQd) / (%ΔY)
A positive YED signals a normal good; luxury goods typically have YED > 1, while necessities show 0 < YED < 1. A negative YED identifies an inferior good, where demand falls as income rises.
正 YED 表明是正常商品；奢侈品的 YED 通常 > 1，必需品的 YED 介于 0 和 1 之间。负 YED 则是低档商品，收入增加时需求量反而下降。
Knowledge of YED helps firms forecast how sales shift over the business cycle and guides long-term production planning.
掌握 YED 有助于企业预测经济周期不同阶段的销售变化，并指导长期生产决策。
3. Cross Elasticity of Demand (XED) | 需求的交叉弹性
Cross elasticity of demand (XED) measures the sensitivity of demand for one good when the price of another good changes.
需求的交叉弹性 (XED) 衡量一种商品的需求对另一种商品价格变化的敏感度。
XED = (%ΔQdA) / (%ΔPB)
A positive XED indicates substitutes (e.g. tea and coffee); a rise in the price of good B increases demand for good A. A negative XED signals complements (e.g. printers and ink cartridges); a rise in the price of good B reduces the demand for good A. An XED close to zero suggests the two goods are unrelated.
正 XED 表示替代品（如茶与咖啡）；B 商品价格上涨会使 A 商品需求增加。负 XED 表示互补品（如打印机和墨盒）；B 价格上涨会减少 A 的需求。XED 接近零则说明两商品不相关。
Firms use XED to assess competitive risk and to plan pricing strategies for product lines that are closely related in consumption.
企业利用 XED 评估竞争风险，并为消费上密切关联的产品线制定定价策略。
4. Price Elasticity of Supply (PES) | 供给的价格弹性
Price elasticity of supply (PES) captures the responsiveness of quantity supplied to a change in the market price.
供给的价格弹性 (PES) 衡量市场价格的变动所引起的供给量反应程度。
PES = (%ΔQs) / (%ΔP)
Supply is price elastic when PES > 1, indicating that firms can adjust output easily. Supply is price inelastic when PES < 1, often because of limited spare capacity or long production lags. The length of the adjustment period is a key determinant: PES tends to be higher in the long run.
若 PES > 1，供给富有价格弹性，说明厂商较容易调整产出。PES < 1 则供给缺乏价格弹性，通常因为闲置产能有限或生产周期长。调整时间长短是关键决定因素：长期 PES 往往更高。
Understanding PES helps predict how quickly markets can respond to demand shifts and how the burden of a tax or subsidy is shared.
理解 PES 有助于预测市场应对需求变化的速度，以及税收或补贴负担的分配方式。
5. Consumer and Producer Surplus | 消费者剩余与生产者剩余
Consumer surplus is the difference between what consumers are willing to pay and what they actually pay, represented by the area below the demand curve and above the equilibrium price.
消费者剩余是消费者愿意支付的最高价格与实际支付价格之间的差额，表现为需求曲线以下、均衡价格以上的区域。
Producer surplus is the difference between the minimum price firms are willing to accept and the market price they receive, shown as the area above the supply curve and below the equilibrium price.
生产者剩余是厂商愿意接受的最低价格与所获市场价格之间的差额，表现为供给曲线以上、均衡价格以下的区域。
Both surpluses are maximised at the free-market equilibrium, where allocative efficiency is achieved (assuming no externalities). The area of a surplus triangle is calculated as:
在无外部性的自由市场均衡点上，两种剩余均达到最大，实现了配置效率。剩余三角形的面积计算公式为：
Area = ½ × base × height
Changes in price, taxes, or subsidies reallocate surplus between consumers and producers, often creating a deadweight loss.
价格、税收或补贴的变化会使剩余在消费者和生产者之间重新分配，并常产生无谓损失。
6. Tax Incidence | 税收归宿
Tax incidence describes how the burden of an indirect tax is split between consumers and producers. The division depends on the relative price elasticities of demand and supply.
税收归宿描述间接税负担如何由消费者和生产者分担，分配取决于需求与供给的相对价格弹性。
The portion borne by consumers is larger when demand is more inelastic than supply. Conversely, producers bear a greater share when supply is more inelastic. A useful approximation for the consumer share per unit tax is:
当需求相对供给更缺乏弹性时，消费者承担的税负更大。反之，若供给更缺乏弹性，生产者负担更重。对每单位税额消费者承担份额的一个近似公式为：
Consumer share = PES / ( |PED| + PES ) × tax
The producer share is the remainder: |PED| / (|PED| s used as an absolute value and both elasticities refer to the original equilibrium point.
生产者份额则为剩余部分：|PED| / (|PED| + PES) × 税额。注意此处 PED 采用绝对值，且两个弹性均在原均衡点计算。
This formula helps evaluate the effectiveness of government policies and the likely welfare effects on different market participants.
这个公式有助于评估政府政策的有效性，及其对市场各方可能的福利影响。
7. The Multiplier Effect | 乘数效应
The multiplier measures the final change in national income resulting from an initial injection into the circular flow of income (e.g. investment, government spending, or exports).
乘数衡量的是向国民收入循环流注入初始支出（如投资、政府支出或出口）后，最终国民收入的变化幅度。
The basic multiplier in a closed economy with no government is expressed using the marginal propensity to consume (MPC). The formula is:
在无政府部门的封闭经济中，基本乘数用边际消费倾向 (MPC) 表示，公式为：
k = 1 / (1 − MPC) = 1 / MPS
In an open economy with government, withdrawals include saving, taxation, and imports. The marginal propensity to withdraw (MPW) is the sum of MPS, marginal rate of taxation, and marginal propensity to import. Hence:
在开放经济并有政府的情况下，漏出包括储蓄、税收和进口。边际漏出倾向 (MPW) 为 MPS、边际税率和边际进口倾向之和，因此：
k = 1 / MPW
national income (ΔY) equals the multiplier times the initial injection (ΔJ): ΔY = k × ΔJ.
MPW 越高，乘数越小；MPW 越低，乘数越大。国民收入总变化量 (ΔY) 等于乘数乘以初始注入 (ΔJ)：ΔY = k × ΔJ。
8. Gross Domestic Product (GDP) | 国内生产总值
Gross domestic product measures the total market value of all final goods and services produced within a country in a given period. The expenditure approach sums four components:
国内生产总值衡量一经济体在一定时期内生产的所有最终商品和服务的市场总价值。支出法将四部分加总：
GDP = C + I + G + (X − M)
where C is household consumption, I is investment, G is government spending, X is exports, and M is imports.
其中 C 为居民消费，I 为投资，G 为政府支出，X 为出口，M 为进口。
To remove the effect of price changes and compare real output, real GDP is calculated using base-year prices:
为消除价格变动的影响并比较实际产出，实际 GDP 用基年价格计算：
Real GDP = Σ (Pbase × Qcurrent)
Economic growth is measured as the annual percentage change in real GDP:
经济增长以实际 GDP 的年变动百分比来衡量：
Growth (%) = [(Real GDPt − Real GDPt-1) / Real GDPt-1] × 100
GDP data underpin policy decisions and international comparisons, though limitations such as income distribution and non-market activity must be noted.
GDP 数据是政策决策和国际比较的基础，但必须留意收入分配和非市场活动等局限性。
9. Measuring Inflation (CPI) | 衡量通货膨胀 (消费者价格指数)
The Consumer Price Index (CPI) tracks changes in the cost of a fixed basket of goods and services representative of household spending. The index is constructed as:
消费者价格指数 (CPI) 追踪一篮子固定、能代表家庭支出的商品和服务成本的变化。其构造公式为：
CPI = (Cost of basket in current year / Cost of same basket in base year) × 100
The annual inflation rate is the percentage increase in the CPI from one period to the next:
年通胀率是 CPI 从前一时期到下一时期的百分比增幅：
Inflation rate (%) = [(CPIt − CPIt-1) / CPIt-1
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