📚 Year 13 WJEC Economics: Formula & Theorem Quick Reference Handbook | WJEC 经济：公式定理速查手册
This concise handbook gathers every key formula, coefficient and graphical theorem you will encounter in Year 13 WJEC Economics. Use it alongside your notes to speed up calculations, link concepts across micro and macro topics, and secure those crucial quantitative marks in your exam.
这本速查手册汇集了 Year 13 WJEC 经济课程中你需要掌握的所有关键公式、系数与图形定理。结合课堂笔记使用，可以帮你快速完成计算，串联微观与宏观知识，稳稳拿下试卷中的量化分。
1. Price Elasticity of Demand (PED) | 需求价格弹性
PED quantifies how responsive the quantity demanded of a good is to a change in its own price. It is always negative due to the law of demand, but we often refer to its absolute value.
PED 衡量一种商品的需求量对其自身价格变化的反应程度。由于需求定律，PED 总是负值，但我们通常使用其绝对值讨论敏感程度。
PED = %Δ Quantity Demanded ÷ %Δ Price
PED = 需求量变动的百分比 ÷ 价格变动的百分比
Interpretation: |PED| > 1 is price elastic (luxuries, many substitutes); |PED| < 1 is price inelastic (necessities, addictive goods); |PED| = 1 is unit elastic.
结果解读：|PED| > 1 为富有弹性（奢侈品、多见替代品）；|PED| < 1 为缺乏弹性（必需品、成瘾性商品）；|PED| = 1 为单位弹性。
2. Income Elasticity of Demand (YED) | 需求收入弹性
YED measures how quantity demanded responds to a change in consumers’ real income. It identifies normal goods (positive YED) and inferior goods (negative YED).
YED 衡量需求量对消费者实际收入变化的反应程度，用于识别正常品（YED 为正）和低档品（YED 为负）。
YED = %Δ Quantity Demanded ÷ %Δ Real Income
YED = 需求量变动的百分比 ÷ 实际收入变动的百分比
For normal goods, YED > 0; luxury goods have YED > 1; necessities have 0 < YED < 1. For inferior goods, YED < 0.
正常品的 YED 大于 0；奢侈品 YED > 1；必需品 YED 介于 0 ～ 1；低档品的 YED 为负值。
3. Cross Elasticity of Demand (XED) | 需求交叉弹性
XED captures the relationship between two goods by showing how the demand for good A changes when the price of good B changes.
XED 通过衡量 B 商品价格变化对 A 商品需求量的影响，揭示两种商品之间的市场关系。
XED = %Δ Quantity Demanded of Good A ÷ %Δ Price of Good B
XED = A 商品需求量变动的百分比 ÷ B 商品价格变动的百分比
Positive XED implies substitutes (e.g. tea and coffee); negative XED implies complements (e.g. printers and ink). The larger the absolute value, the stronger the relationship.
XED 为正值说明两商品互为替代品（如茶与咖啡）；负值说明是互补品（如打印机与墨盒）。绝对值越大，关联性越强。
4. Price Elasticity of Supply (PES) | 供给价格弹性
PES indicates how responsive the quantity supplied is to a change in the market price. It is almost always positive.
PES 反映供给量对市场价格变化的反应敏感度，其数值在绝大多数情况下为正。
PES = %Δ Quantity Supplied ÷ %Δ Price
PES = 供给量变动的百分比 ÷ 价格变动的百分比
Key determinants include time period (supply is more elastic in the long run), spare production capacity, stock levels and the ease of factor substitution.
关键决定因素有时间跨度（长期供给更富有弹性）、闲置产能、库存水平及要素替代的难易程度。
5. The Multiplier Effect | 乘数效应
The multiplier shows how an initial injection into the circular flow leads to a greater final change in national income. It arises from successive rounds of spending.
乘数效应表明，向经济循环流注入的一笔初始支出会通过多轮次消费，最终带来更大的国民收入变动。
Simple closed-economy multiplier (no government or foreign sector):
简单封闭经济乘数（无政府部门和对外贸易）：
k = 1 ÷ (1 – MPC) = 1 ÷ MPS
k = 1 ÷ (1 – MPC) = 1 ÷ MPS
Government spending multiplier = 1 ÷ (1 – MPC). Tax multiplier = –MPC ÷ MPS. Balanced-budget multiplier = 1.
政府支出乘数 = 1 ÷ (1 – MPC)；税收乘数 = –MPC ÷ MPS；平衡预算乘数恒等于 1。
Open-economy multiplier with proportional tax (t) and marginal propensity to import (MPM):
开放经济下，考虑比例税 t 和边际进口倾向 MPM 的乘数：
k = 1 ÷ [1 – MPC(1 – t) + MPM]
k = 1 ÷ [1 – MPC(1 – t) + MPM]
6. Aggregate Demand (AD) Components | 总需求的构成
Aggregate demand is the total spending on domestic goods and services at a given price level. Its components directly feed into the circular flow model and the AD curve.
总需求是在既定物价水平下，对国内商品与服务的总支出。其各构成部分直接体现在循环流模型和 AD 曲线中。
AD = C + I + G + (X – M)
AD = 消费 + 投资 + 政府支出 + (出口 – 进口)
C: household consumption (largest component); I: firms’ planned investment; G: government current and capital spending; X – M: net exports (exports minus imports).
C 为家庭消费（最大占比）；I 为厂商计划投资；G 为政府经常性和资本性支出；X – M 为净出口（出口减进口）。
7. The Output Gap & Macroeconomic Equilibrium | 产出缺口与宏观经济均衡
The output gap compares actual national output with the economy’s potential (full-capacity) output. A negative output gap indicates spare capacity and unemployment; a positive gap signals overheating and inflationary pressure.
产出缺口对比了实际国民产出与潜在（充分产能）产出。负产出缺口意味着存在闲置资源和失业；正产出缺口则表明经济过热并带来通胀压力。
Output Gap = Actual GDP – Potential GDP
产出缺口 = 实际 GDP – 潜在 GDP
Often expressed as a percentage of potential GDP: Output Gap (%) = [(Actual – Potential) ÷ Potential] × 100.
常表示为潜在 GDP 的百分比：产出缺口 (%) = [(实际 GDP – 潜在 GDP) ÷ 潜在 GDP] × 100。
Short-run equilibrium occurs where AD = SRAS; long-run equilibrium requires AD = LRAS. The classical LRAS is vertical, while the Keynesian LRAS has a horizontal section.
短期均衡在 AD = SRAS 时实现；长期均衡要求 AD = LRAS。古典 LRAS 是垂直的，而凯恩斯 LRAS 包含水平段。
8. Consumer Price Index (CPI) & Inflation | 消费价格指数与通货膨胀
The CPI tracks changes in the price of a representative basket of goods and services purchased by households. It provides the primary measure of inflation in the UK.
CPI 追踪一篮子代表性家庭消费品和服务价格的变化，是英国测量通货膨胀的主要指标。
CPI = (Cost of Basket in Current Year ÷ Cost of Basket in Base Year) × 100
CPI = (当年一篮子成本 ÷ 基年一篮子成本) × 100
The annual inflation rate is then calculated as:
年通胀率则由 CPI 的变动率计算得出：
Inflation Rate = (CPIₜ – CPIₜ₋₁) ÷ CPIₜ₋₁ × 100
通货膨胀率 = (本期 CPI – 上期 CPI) ÷ 上期 CPI × 100
WJEC candidates should also be familiar with RPI (includes mortgage interest) and the GDP deflator = (Nominal GDP ÷ Real GDP) × 100.
考生还须熟悉 RPI（含房贷利息）以及 GDP 平减指数 = (名义 GDP ÷ 实际 GDP) × 100。
9. Unemployment Rate Calculation | 失业率计算
The unemployment rate expresses the number of people actively seeking work as a proportion of the labour force. The labour force comprises all employed and unemployed individuals who are willing and able to work.
失业率表示积极寻找工作的人数占劳动力人口的比例。劳动力人口包含所有有工作能力且愿意工作的就业者与失业者总和。
Unemployment Rate = (Number of Unemployed ÷ Labour Force) × 100
失业率 = (失业人数 ÷ 劳动力总数) × 100
Be aware of the claimant count (those receiving Jobseeker’s Allowance) and the ILO/LFS measure (survey-based, includes those actively seeking work). Both are used in WJEC data-response questions.
注意申领人数（领取求职津贴者）和国际劳工组织 / 劳动力调查标准下的失业统计（基于调查、涵盖积极求职者）。WJEC 数据分析题常同时涉及这两种口径。
10. Exchange Rate & Terms of Trade | 汇率与贸易条件
An exchange rate expresses the price of one currency in terms of another. Appreciation means the currency has strengthened; depreciation means it has weakened.
汇率表示一种货币以另一种货币计价的价格。升值表示本币走强，贬值表示本币走弱。
The terms of trade index measures a country’s export prices relative to its import prices, indicating the volume of imports that can be bought with a given volume of exports.
贸易条件指数衡量一国出口价格相对于进口价格的比值，反映一定出口量能够换回的进口量。
Terms of Trade Index = (Index of Export Prices ÷ Index of Import Prices) × 100
贸易条件指数 = (出口价格指数 ÷ 进口价格指数) × 100
An improvement in the terms of trade (index rising) raises living standards in the short run but can hurt export competitiveness. A deterioration does the opposite.
贸易条件改善（指数上升）短期内提升生活水平，但可能削弱出口竞争力；贸易条件恶化则反之。
11. Comparative Advantage & Opportunity Cost | 比较优势与机会成本
Comparative advantage is the principle that countries should specialise in producing goods where they have the lowest opportunity cost, even if one country holds an absolute advantage in all goods.
比较优势原理指出，即便一国在所有商品生产上都拥有绝对优势，各国仍应专门生产机会成本最低的商品并开展贸易。
Opportunity cost of producing one unit of good X is the quantity of good Y foregone: OC = ΔY ÷ ΔX.
生产一单位 X 商品的机会成本是所放弃的 Y 商品数量：机会成本 = Y 的减少量 ÷ X 的增加量。
Opportunity Cost of Good X = Units of Good Y Sacrificed ÷ Units of Good X Gained
X 商品的机会成本 = 牺牲的 Y 数量 ÷ 获得的 X 数量
To determine the pattern of trade, compare pre-trade opportunity costs for each country. The country with the lower opportunity cost in X should specialise in X and export it.
确定贸易格局时，对比各国生产同一商品的机会成本；机会成本较低的国家应专门生产并出口该商品。
12. Lorenz Curve & Gini Coefficient | 洛伦兹曲线与基尼系数
The Lorenz curve plots the cumulative percentage of income (or wealth) against the cumulative percentage of households. The diagonal line represents perfect equality.
洛伦兹曲线描绘了家庭累计百分比与收入累计百分比之间的关系。对角线代表绝对均等。
The Gini coefficient quantifies the degree of inequality by comparing the area between the line of equality and the Lorenz curve to the total area under the line of equality.
基尼系数通过计算洛伦兹曲线与绝对均等线之间的面积相对于均等线下方面积的比例，来量化不平等程度。
Gini Coefficient = A ÷ (A + B)
基尼系数 = A ÷ (A + B)
A is the area between the perfect equality line and the Lorenz curve; B is the area below the Lorenz curve. A Gini value of 0 represents perfect equality, while 1 represents maximum inequality.
A 为绝对均等线与洛伦兹曲线之间的面积，B 为洛伦兹曲线下方的面积。基尼系数为 0 表示绝对均等，1 表示绝对不平等。
In WJEC exams you may be asked to interpret shifts of the Lorenz curve or changes in the Gini coefficient caused by policies such as progressive taxation or welfare transfers.
WJEC 试卷中可能会要求你分析累进税制或福利转移等政策如何导致洛伦兹曲线移动或基尼系数变化。
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