📚 A-Level OCR Economics: Formula Summary Handbook | A-Level OCR 经济学公式汇总手册
This article provides a comprehensive compilation of the key formulas required for the OCR A-Level Economics syllabus. From microeconomic foundations like elasticity and costs to macroeconomic tools such as the multiplier and terms of trade, mastering these quantitative relationships is essential for exam success. Every formula is paired with a clear explanation in both English and Chinese, making it an ideal last-minute revision resource.
本文全面汇编了OCR A-Level经济学大纲所要求的关键公式。无论是需求弹性、成本曲线等微观基础，还是乘数效应、贸易条件等宏观工具，掌握这些量化关系对于考试成功至关重要。每个公式均配有清晰的中英双语解释，是理想的考前速查手册。
1. Elasticity Formulas | 弹性公式
Price, income, cross and supply elasticities measure how responsive quantity is to a change in a determinant. Each is calculated as the percentage change in quantity divided by the percentage change in the trigger variable. The midpoint (arc) method is preferred when working with discrete data.
价格弹性、收入弹性、交叉弹性和供给弹性衡量数量对某一决定因素变动的反应程度。每种弹性的计算方法是数量的百分比变动除以触发变量的百分比变动。在使用离散数据时，通常采用中点（弧）法。
The four core elasticity formulas are displayed below. For the midpoint approach, Q and P in the denominator are replaced by their averages: Qavg = (Q₁+Q₂)/2 and Pavg = (P₁+P₂)/2.
四种核心弹性公式展示如下。中点法需将分母中的 Q 和 P 替换为平均值：Qavg = (Q₁+Q₂)/2，Pavg = (P₁+P₂)/2。
PED = %ΔQd / %ΔP
YED = %ΔQd / %ΔY
XED = %ΔQdA / %ΔPB
PES = %ΔQs / %ΔP
The following table summarises how the absolute value of PED relates to total revenue and the shape of the demand curve.
下表总结了 PED 绝对值与总收益及需求曲线形状的关系。
| |PED| Value | Description | Effect on Total Revenue if P rises |
|---|---|---|
| > 1 | Elastic | TR falls |
| = 1 | Unit elastic | TR unchanged |
| < 1 | Inelastic | TR rises |
2. Costs, Revenue and Profit | 成本、收益与利润
Understanding a firm’s cost structure and revenue streams is fundamental to the theory of the firm. All short-run cost curves are derived from the underlying total fixed cost (TFC) and total variable cost (TVC).
理解企业的成本结构与收入来源是厂理论的基础。所有短期成本曲线均源自总固定成本（TFC）和总可变成本（TVC）。
Total Cost (TC) = TFC + TVC
Average Fixed Cost (AFC) = TFC / Q
Average Variable Cost (AVC) = TVC / Q
Average Total Cost (ATC) = TC / Q = AFC + AVC
Marginal Cost (MC) = ΔTC / ΔQ
On the revenue side, total revenue is price multiplied by quantity sold. In perfect competition, the firm is a price-taker, so marginal revenue equals the constant price. In imperfect markets, MR falls faster than AR.
在收入方面，总收益为价格乘以销量。在完全竞争市场中，厂商是价格接受者，因此边际收益恒等于价格。在不完全市场中，边际收益下降速度快于平均收益。
Total Revenue (TR) = P × Q
Average Revenue (AR) = TR / Q = P
Marginal Revenue (MR) = ΔTR / ΔQ
Economic profit is the surplus after covering all opportunity costs. Normal profit occurs when TR = TC, meaning the firm is just covering its explicit and implicit costs. Supernormal profit arises when TR > TC.
经济利润是覆盖全部机会成本后的盈余。正常利润出现在 TR = TC 时，即企业刚好弥补显性成本与隐性成本。超常利润则出现在 TR > TC 时。
Profit = TR − TC
3. Profit Maximisation and Efficiency | 利润最大化与效率
Profit maximisation is the standard objective assumed for firms. In addition, the syllabus requires candidates to recognise other targets and the conditions for allocative and productive efficiency.
利润最大化是假设的企业标准目标。此外，大纲要求考生了解其他目标以及配置效率与生产效率的条件。
The profit-maximising rule sets output where marginal cost equals marginal revenue. This applies irrespective of market structure.
利润最大化规则在边际成本等于边际收益的产量处成立。无论市场结构如何，该条件均适用。
Profit maximisation: MC = MR
Revenue maximisation occurs where MR falls to zero, while sales (output) maximisation pushes production to the point where average revenue equals average cost, yielding normal profit only.
收益最大化发生在 MR 降至零时，而销量（产量）最大化则将生产推至平均收益等于平均成本处，此时仅获得正常利润。
Revenue maximisation: MR = 0
Sales maximisation: AR = ATC
Allocative efficiency is achieved when the price consumers are willing to pay matches the marginal cost of production. Productive efficiency occurs when output is produced at minimum average total cost, which corresponds to the point where MC = ATC.
配置效率在消费者愿意支付的价格等于生产的边际成本时实现。生产效率在产出以最低平均总成本生产时达到，这对应 MC = ATC 的交点。
Allocative efficiency: P = MC
Productive efficiency: ATC is minimised (MC = ATC)
4. Market Concentration | 市场集中度
Concentration ratios and the Herfindahl-Hirschman Index (HHI) are used to assess the degree of competition within an industry. A higher value indicates a more concentrated, potentially less competitive market.
集中度比率和赫芬达尔-赫希曼指数（HHI）用于评估行业内的竞争程度。数值越高，表明市场越集中，竞争可能越弱。
Concentration Ratio (CRn) = (Total sales of n largest firms / Total market sales) × 100
CR₃ and CR₅ are the most common variants. An alternative measure that captures the whole size distribution is the HHI, which squares each firm’s market share (as a percentage) and sums the squares.
CR₃ 和 CR₅ 是最常用的形式。另一种反映整体规模分布的方法是 HHI，它将每家企业的市场份额（百分比）平方后求和。
HHI = Σ(si)², where si is the market share % of firm i
For example, a market with two equal-sized firms has an HHI of 50² + 50² = 5,000. Values above 2,000 typically signal high concentration.
例如，一家由两家同等规模企业组成的市场，其 HHI 为 50² + 50² = 5,000。超过 2,000 的数值通常意味着高度集中。
5. National Income Aggregates | 国民收入总量
National income accounting provides the monetary value of all final goods and services produced in an economy. These aggregates form the basis for measuring economic growth and living standards.
国民收入核算提供了经济中所有最终产品与服务的货币价值。这些总量是衡量经济增长和生活水平的基础。
The expenditure method sums the key spending injections. GDP per capita divides gross domestic product by the population to give an average output per person.
支出法将主要的支出注入进行相加。人均 GDP 将国内生产总值除以人口，得到人均产出。
GDP (expenditure approach) = C + I + G + (X − M)
GDP per capita = GDP / Population
Gross National Income (GNI) adjusts GDP for net income flows from abroad, capturing the income actually earned by residents.
国民总收入（GNI）将 GDP 根据国外净收入流进行调整，反映居民实际获得的收入。
GNI = GDP + Net primary income from abroad
6. Nominal and Real Values | 名义值与实际值
Inflation distorts inter-year comparisons, so economists deflate nominal figures using a price index. The two most common tools are the GDP deflator and the Consumer Price Index (CPI).
通货膨胀会使跨年比较失真，因此经济学家采用价格指数将名义数据平减。两种最常用的工具是 GDP 平减指数和消费者物价指数（CPI）。
Real GDP = (Nominal GDP / GDP Deflator) × 100
The CPI measures the average change in prices paid by households for a fixed basket of goods. The inflation rate is then the percentage change in the CPI.
CPI 衡量家庭为固定一篮子商品支付的价格的平均变化。通货膨胀率即 CPI 的百分比变化。
CPI =
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