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Supply and Demand Analysis

供需分析

Overview

概述

Supply and demand is the foundational model for price determination in markets. It explains how prices emerge from the interaction of buyers (demand) and sellers (supply), and how external shocks or policy interventions shift the equilibrium.
供需模型是市场价格决定的基础模型。它解释了价格如何从买方（需求）和卖方（供给）的互动中产生，以及外部冲击或政策干预如何改变均衡状态。

Framework

框架

IRON LAW: Shift vs Movement Along the Curve

A change in PRICE causes movement ALONG the curve (quantity changes).
A change in OTHER FACTORS (income, costs, preferences, technology) causes
the entire curve to SHIFT.

"Demand increased" means the demand CURVE shifted right (more quantity
at every price), NOT that quantity demanded increased due to a price drop.
Confusing these is the #1 error in supply-demand analysis.
IRON LAW: Ceteris Paribus — Change One Thing at a Time

When analyzing a shift, hold everything else constant. If both supply
and demand shift simultaneously, the effect on price or quantity (or both)
becomes ambiguous. Analyze each shift separately first, then combine.
IRON LAW: Shift vs Movement Along the Curve

A change in PRICE causes movement ALONG the curve (quantity changes).
A change in OTHER FACTORS (income, costs, preferences, technology) causes
the entire curve to SHIFT.

"Demand increased" means the demand CURVE shifted right (more quantity
at every price), NOT that quantity demanded increased due to a price drop.
Confusing these is the #1 error in supply-demand analysis.
IRON LAW: Ceteris Paribus — Change One Thing at a Time

When analyzing a shift, hold everything else constant. If both supply
and demand shift simultaneously, the effect on price or quantity (or both)
becomes ambiguous. Analyze each shift separately first, then combine.

Demand Curve Shifters (non-price factors)

需求曲线移动因素（非价格因素）

  • Consumer income (normal goods: ↑income → ↑demand; inferior goods: opposite)
  • Prices of related goods (substitutes, complements)
  • Consumer preferences and tastes
  • Population / number of buyers
  • Expectations of future prices
  • 消费者收入（正常商品：收入增加→需求增加；劣等商品则相反）
  • 相关商品价格（替代品、互补品）
  • 消费者偏好与品味
  • 人口/买家数量
  • 对未来价格的预期

Supply Curve Shifters (non-price factors)

供给曲线移动因素（非价格因素）

  • Input costs (raw materials, labor, energy)
  • Technology improvements
  • Number of sellers
  • Government policy (taxes, subsidies, regulations)
  • Expectations of future prices
  • 投入成本（原材料、劳动力、能源）
  • 技术进步
  • 卖家数量
  • 政府政策（税收、补贴、监管）
  • 对未来价格的预期

Analysis Steps

分析步骤

  1. Define the market: What good? What geographic/time scope?
  2. Identify the shock: What changed? Is it demand-side or supply-side?
  3. Determine shift direction: Does the curve shift left (decrease) or right (increase)?
  4. Find new equilibrium: Where do the new curves intersect?
  5. State the result: What happens to price and quantity?
  1. 定义市场：是什么商品？地理/时间范围是？
  2. 识别冲击：什么发生了变化？是需求端还是供给端的变化？
  3. 确定移动方向：曲线是向左移动（减少）还是向右移动（增加）？
  4. 找到新均衡：新的曲线交点在哪里？
  5. 说明结果：价格和数量会发生什么变化？

Policy Analysis

政策分析

InterventionEffect
Price ceiling (below eq.)Shortage: Qd > Qs, black markets, quality reduction
Price floor (above eq.)Surplus: Qs > Qd, waste, inefficiency
Per-unit taxSupply shifts left by tax amount, price rises (split between buyer and seller based on elasticity)
SubsidySupply shifts right by subsidy amount, price falls
TariffImport supply shifts left, domestic price rises
干预措施影响
价格上限（低于均衡价）短缺：需求量>供给量，黑市出现，质量下降
价格下限（高于均衡价）过剩：供给量>需求量，浪费，低效
单位税供给曲线向左移动税额幅度，价格上升（买卖双方的税负分摊取决于弹性）
补贴供给曲线向右移动补贴额幅度，价格下降
关税进口供给曲线向左移动，国内价格上升

Output Format

输出格式

markdown
undefined
markdown
undefined

Supply-Demand Analysis: {Market}

Supply-Demand Analysis: {Market}

Market Definition

Market Definition

  • Good: ...
  • Scope: ...
  • Good: ...
  • Scope: ...

Shock Identification

Shock Identification

  • Event: {what changed}
  • Affected curve: Demand / Supply
  • Direction: Shift left / right
  • Mechanism: {why this shifts the curve}
  • Event: {what changed}
  • Affected curve: Demand / Supply
  • Direction: Shift left / right
  • Mechanism: {why this shifts the curve}

Equilibrium Impact

Equilibrium Impact

BeforeAfterChange
Price$X$X↑/↓
QuantityX unitsX units↑/↓
BeforeAfterChange
Price$X$X↑/↓
QuantityX unitsX units↑/↓

Analysis

Analysis

{Detailed explanation with diagram description}
undefined
{Detailed explanation with diagram description}
undefined

Examples

示例

Correct Application

正确应用

Scenario: Effect of a new EV subsidy on Taiwan's electric car market
  • Shock: Government offers NT$150K subsidy per EV purchase
  • Affected curve: Demand shifts RIGHT (effectively lowers price for buyers → more quantity demanded at every market price)
  • Result: Price rises slightly (sellers capture part of subsidy), quantity increases significantly
  • Note: Supply curve does not shift — the subsidy goes to buyers, not manufacturers ✓
场景： 新电动汽车补贴对台湾电动汽车市场的影响
  • 冲击：政府为每辆电动汽车提供15万新台币的补贴
  • 受影响曲线：需求曲线向右移动（实际上降低了买家的购车成本→每个市场价格下的需求量都增加）
  • 结果：价格小幅上涨（卖家获得部分补贴），销量大幅增加
  • 注意：供给曲线并未移动——补贴是给买家的，而非制造商 ✓

Incorrect Application

错误应用

  • "EV prices dropped so demand increased" → This is movement ALONG the demand curve, not a shift. Demand (the curve) didn't increase — quantity demanded increased. Violates Iron Law.
  • Analyzed EV subsidy + chip shortage simultaneously without separating → Ambiguous result. Must analyze each shift separately first. Violates ceteris paribus.
  • "电动汽车价格下降，所以需求增加" → 这是需求曲线沿曲线移动，而非曲线平移。需求（曲线）并未增加——只是需求量增加了。违反了铁律。
  • 同时分析电动汽车补贴和芯片短缺而未分开处理→结果模糊。必须先分别分析每一种变动，再结合起来。违反了其他条件不变原则。

Gotchas

注意事项

  • Elasticity determines burden: For taxes/subsidies, the MORE INELASTIC side bears more of the burden. If demand is inelastic (necessities), consumers bear most of a tax increase.
  • Short run vs long run: Supply is more elastic in the long run (firms can enter/exit, build capacity). Short-run analysis may overestimate price changes.
  • Not all markets fit: Supply-demand assumes competitive markets. Monopolies, oligopolies, and markets with externalities need different models.
  • "Demand" is not "want": Demand requires both willingness AND ability to pay. A billion people wanting iPhones is not demand if they can't afford one.
  • 弹性决定税负分摊：对于税收/补贴，弹性越低的一方承担的税负越多。如果需求缺乏弹性（必需品），消费者将承担大部分税收增长的成本。
  • 短期 vs 长期：长期供给弹性更高（企业可以进入/退出市场，扩大产能）。短期分析可能高估价格变动幅度。
  • 并非所有市场都适用：供需模型假设市场是竞争性的。垄断、寡头垄断以及存在外部性的市场需要不同的模型。
  • “需求”不等于“想要”：需求需要同时具备意愿和支付能力。十亿人想要iPhone但买不起，这不构成需求。

References

参考资料

  • For elasticity calculations, see 
    references/elasticity.md
  • For welfare analysis (consumer/producer surplus), see 
    references/welfare-analysis.md
  • 弹性计算请参考 
    references/elasticity.md
  • 福利分析（消费者/生产者剩余）请参考 
    references/welfare-analysis.md