A-Level经济 图表分析 供需移动 弹性 福利
A-Level 经济的卷面上有一类题看起来最省时间，就是给一张空白坐标让你画。分数却常常丢在这里。下面按卷面实际出现的十几种图，逐一拆开坐标轴、曲线移动方向与福利区域的读法，并把评分标准里那几分到底给在哪一步写清楚。文中的算例都标了假设条件，官方数据标了来源与发布日期。
Diagram questions look like the quickest marks on an A-Level Economics paper, and they are also where a surprising number of marks disappear. The sections below work through the diagrams that actually appear in exams, from axes and curve shifts to welfare areas, and set out where each mark in the mark scheme is earned; numbers described as assumptions are illustrative, while official figures carry their source and release date.
图表题的给分口径与分值分布
带图的题在 A-Level 经济里通常按 4 分给，评分标准把这 4 分拆成坐标轴与初始曲线、移动方向与标注、新的均衡点、一句解释四个动作。Edexcel 9EC0 三卷各 2 小时 100 分，Paper 1 覆盖 Theme 1 与 Theme 3，Paper 2 覆盖 Theme 2 与 Theme 4，两卷的 Section A 短答题里就会出现这种小图。AQA 7136 的三卷各 2 小时 80 分，数据回应题同样要求先画图再读变化。很多学生以为画出两条线就算完成，实际上没写新均衡价格与数量的那一行，最多给到 2 分。阅卷时最常见的卷面是曲线方向对了，标签漏了，于是丢掉结构分的后一半。把四个动作先写在草稿纸上再落笔，比画完回头补说明更省时间。
A four-mark diagram question is marked in steps rather than as one picture: axes and the initial curve, the shift with its label, the new equilibrium, and a sentence of explanation. Edexcel 9EC0 runs three papers of 2 hours and 100 marks, with Themes 1 and 3 on Paper 1 and Themes 2 and 4 on Paper 2, and its short-answer sections are where these small diagrams turn up; AQA 7136 runs three papers of 2 hours and 80 marks with data-response questions that ask for the diagram first and the reading second. Candidates who draw a correct shift but never state the new price and quantity usually stop at two marks.
纵轴写 price 还是 price level
微观题的纵轴是价格，宏观题的纵轴是价格水平，这两个词在评分标准里不能互换。Pearson 的阅卷报告点过一个具体现象，Paper 2 的四分图题里，纵轴写成 price 而不是 price level 的答卷拿不到第 4 分。横轴同样要看清楚，微观题写数量 quantity，宏观题写实际产出 real GDP。很多学生以为单位只是形式问题，实际上它决定了这张图讲的是单个市场还是整个经济。给曲线加标签也别省，AD、SRAS、LRAS 写在曲线旁边比写在图下方更容易被找到。坐标轴不写货币单位时，改用价格指数更稳妥，通胀期间尤其如此。
Micro questions put price on the vertical axis, while macro questions put the price level there, and the two labels are not interchangeable in a mark scheme. Pearson's examiner reports single this out: on a four-mark Paper 2 diagram a vertical axis labelled price rather than price level costs the final mark even when the shift itself is right; the horizontal axis carries the same trap, since quantity belongs to a single market and real GDP to the whole economy. Labelling each curve beside the line, rather than in a caption underneath, also makes the script easier to credit.
沿曲线移动与整条曲线平移
需求曲线上的一点换到另一点，只有一种原因，就是这件商品自己的价格变了。收入、偏好、替代品价格、互补品价格、人口与预期变动，动的是整条曲线。学生最常把两者混在一起的地方，是看到某商品涨价就画需求曲线左移。涨价沿曲线向上移动，成交量减少，曲线本身没动。阅卷时看到移动方向标反、文字里却写着价格变化的答卷，通常只给 1 分。画平移时用虚线与实线各一条，并在虚线旁标 D1、实线旁标 D2，比两条一样的实线清楚得多。
A movement along a demand curve has exactly one cause, namely a change in the good's own price. Income, tastes, substitute prices, complement prices, population and expectations move the whole curve instead. Many candidates reach for a leftward shift whenever they read that a price has risen, which is the wrong diagram: a higher own price moves the point up the existing curve and leaves the curve where it was; two distinctly drawn lines, one dashed and labelled D1 and the other solid and labelled D2, leave no room for that confusion.
需求平移的五个非价格因素
需求左移还是右移，先判断因素影响的是支付意愿还是支付能力。可支配收入上升，正常品需求右移，低档品反而左移，这一点在评估题里常被忽略。替代品涨价，本商品需求右移；互补品涨价，本商品需求左移，方向正好相反。人口结构变化影响的是市场整体规模，老年人口占比上升会推高医疗与护理类需求。预期通胀上升时，耐用品的需求往往提前释放。常见的错误是把广告支出算进价格因素，它改变的是偏好，属于整条曲线平移。
Deciding the direction of a demand shift starts with one question: does the factor change willingness to pay or the ability to pay? A rise in disposable income shifts normal goods right and inferior goods left, a distinction that evaluation marks reward, while a higher price for a substitute shifts demand right and a higher price for a complement shifts it left. Advertising changes preferences, so it shifts the curve rather than moving along it.
供给平移与产能上限
供给右移意味着在每一个价格上愿意并且能够卖出的数量都增加了。成本下降、技术进步、企业数量增加、补贴与减税都会让供给右移，原材料涨价、工资上调、间接税开征则让供给左移。产能是另一条线索，短期内厂房与设备固定，供给曲线在接近满产时变得很陡，这一段正好解释急单为什么要加价。2026 年 4 月 1 日起，英国 21 岁及以上最低工资为每小时 12.71 英镑（gov.uk），把它当作工资成本上升的例子时，要说明受影响的是劳动密集行业。把产能段画成折线而不是直线，能省下解释的篇幅。
A rightward shift of supply means more is offered at every price, which is what lower costs, better technology, more firms, subsidies and tax cuts all produce, while higher input prices, wage settlements and indirect taxes push the curve the other way. Capacity explains the shape: with plant fixed in the short run supply becomes steep as output approaches full capacity, and that steep segment is why urgent orders command a premium. From 1 April 2026 the National Living Wage for those aged 21 and over is 12.71 pounds an hour on gov.uk, a usable example of a wage-cost shock in labour-intensive industries.
均衡、短缺与过剩的调整方向
市场价格高于均衡价格时，供给量大于需求量，出现过剩，库存上升会推着价格往下走。价格低于均衡时，需求量大于供给量，出现短缺，排队与黄牛价就是短缺的可见形式。调整过程写在图上是价格向均衡点移动，成交量沿着两条曲线收敛。阅卷时经常看到只写短缺、不写调整方向的答卷，那一分往往拿不到。把过剩与短缺分开画两张小图，比在一张图上叠两条箭头更清楚。均衡点建议用小圆点标出，再写一句新价格与新数量，读者一眼能找到结论。
Above the equilibrium price, quantity supplied exceeds quantity demanded, stock builds up and the surplus pushes the price down. Below it, demand exceeds supply and the shortage shows up as queues, rationing or a resale premium. In diagram terms the adjustment is a price moving back to the intersection with the traded quantity settling there; simply naming the shortage without saying which way the price then moves is a routine way to lose the mark.
需求价格弹性在斜率上怎么读
弹性不是斜率，但在同一张图上，越平缓的需求曲线在给定价格附近弹性越大。算例可以自己设定并注明是假设：价格从 2.00 英镑升到 2.20 英镑，涨幅 10%，销量从 2000 件降到 1700 件，降幅 15%，弹性 1.5。原来的收入是 4000 英镑，涨价后变成 3740 英镑，总收入下降正说明需求富有弹性。这一句判断在评估题里比背定义值钱。常见的错误是只看斜率就下结论，忘了斜率相同、位置不同的两条曲线弹性并不一样。图上的价格区间要标清楚，弹性只在那个区间里成立。
Elasticity is not slope, yet on a single diagram the flatter demand curve is the more elastic one around a given price. A worked case makes the link concrete: a price rise from 2.00 to 2.20 pounds is 10 per cent, sales falling from 2000 to 1700 units is 15 per cent, so elasticity is 1.5; revenue moves from 4000 to 3740 pounds, and that fall in total revenue is the evidence of elastic demand which evaluation marks look for. Reading elasticity straight off the slope, without checking position, is where the reasoning usually breaks.
间接税归宿与福利损失三角
对供给方征收从量税，供给曲线垂直上移一个税距，新的均衡价格上升、数量下降。税负由谁承担取决于两条曲线的相对弹性，需求越缺乏弹性，消费者承担的份额越大。图上夹在原均衡与新均衡之间的那个三角形是福利损失，两侧分别是消费者剩余的损失、生产者剩余的损失与政府税收收入。软饮料行业税从 2018 年 4 月 6 日起实施，含糖 5 至 8 克每 100 毫升每升 18 便士，8 克以上每升 24 便士（gov.uk），这是把外部成本写进价格的现成例子。学生最常把税距画成从原价格出发的水平线段，方向一错整张图都作废。税收收入那块用四边形标出，别和福利三角混在一起。
A specific tax on sellers shifts supply vertically upward by the tax per unit, raising the equilibrium price and lowering the quantity traded. Who bears the burden depends on relative elasticities, and the less elastic side absorbs more of it. The triangle between the old and new quantity is the deadweight loss, flanked by the lost consumer surplus, the lost producer surplus and the tax revenue collected; the Soft Drinks Industry Levy, in force since 6 April 2018 at 18 pence per litre for drinks with 5 to 8 grams of sugar per 100 millilitres and 24 pence above that on gov.uk, is a working example of a tax placed on an external cost.
负外部性的两张图
负外部性的标准画法有三条线：边际私人成本 MPC、边际社会成本 MSC 与边际私人收益 MPB，MSC 画在 MPC 上方，垂直距离就是边际外部成本。市场均衡落在 MPC 与 MPB 的交点，社会最优落在 MSC 与 MPB 的交点，前者产量偏大。两条产量之间的三角区域是福利损失，用 MSC 与 MPB 之间的面积衡量。很多学生以为画出 MSC 在 MPC 上方就够了，实际上没有标出两个产量点，福利损失那一分给不出去。正外部性的图把三条线换成 MSB 在 MPB 上方，方向整个反过来。补贴图与税收图是同一套结构，只是曲线移动方向相反。
The negative externality diagram needs three lines: marginal private cost, marginal social cost positioned above it by the marginal external cost, and marginal private benefit. The market settles where private cost meets private benefit, while the social optimum sits where social cost meets the same benefit curve, at a lower output. The triangle bounded by those two quantities, measured between the social cost and benefit lines, is the welfare loss; leaving out the two quantity labels is enough to lose the welfare mark even when all three lines are correct.
AD 平移、SRAS 与 LRAS 的位置
宏观图的纵轴是价格水平，横轴是实际产出。AD 由消费、投资、政府支出与净出口四块组成，任何一块变动都会让 AD 平移。SRAS 向上倾斜，短期内工资与部分成本粘住，所以价格上升会带来产出增加。LRAS 垂直画在充分产能处，位置由要素数量与生产率决定。需求拉动型通胀在图上表现为 AD 右移沿着 SRAS 上移，价格上涨、产出增加；成本推动型是 SRAS 左移，价格上涨、产出下降。两条路径的结论相反，答卷里要把是哪一条写清楚。产出缺口就是实际产出与 LRAS 之间的水平距离。
On a macro diagram the vertical axis is the price level and the horizontal axis is real output. Aggregate demand is the sum of consumption, investment, government spending and net exports, and a change in any component shifts the whole curve. Short-run aggregate supply slopes upward because wages and some costs are sticky, so a higher price level brings extra output, whereas long-run aggregate supply is vertical at full capacity and fixed by the quantity and productivity of factors; demand-pull inflation shows up as a rightward shift of aggregate demand and cost-push as a leftward shift of short-run supply.
乘数对 AD 平移幅度的影响
乘数把初始支出变化放大成更大的产出变化，公式是 1 除以 1 减边际消费倾向。假设边际消费倾向为 0.75，乘数就是 4，政府支出增加 20 亿英镑，理论上产出增加 80 亿英镑。这个算例要写明是假设值，因为真实乘数取决于税负、进口漏出与闲置产能。图上表现为同一条 AD 右移，乘数大的情形平移幅度更大，随之而来的价格上升也更快。评估题里可以接着写产能接近上限时，乘数效应更多体现在价格而不是产出上。把两段 AD 画成不同长度的箭头并列，读者一眼能看出区别。
The multiplier turns an initial change in spending into a larger change in output, and it equals one divided by one minus the marginal propensity to consume. Take a marginal propensity to consume of 0.75: the multiplier is 4, so an extra 2 billion pounds of government spending raises output by 8 billion in theory, though those are assumed values since the real multiplier depends on tax, import leakage and spare capacity. On the diagram a larger multiplier is simply a bigger rightward shift of aggregate demand, with a steeper price response once capacity gets tight.
关税图与汇率图的读法
关税图先把世界价格画成一条水平线，它代表小国假设下可以无限量进口的价格。关税加在世界价格之上，国内价格上移，国内产量增加、进口量减少。图上要标出四块面积：生产者剩余增加、消费者剩余减少、政府关税收入与两块福利损失。汇率图的纵轴是汇率，横轴是货币数量，出口增加带来外币供给增加，本币升值。学生最常把升值与出口竞争力下降的因果关系写反，图上方向对、文字写反一样扣分。把两块福利三角分别标上字母，比只画阴影更容易拿分。
A tariff diagram begins with the world price as a horizontal line, the price at which imports are available without limit under the small-country assumption. The tariff lifts the domestic price above it, so domestic output rises and imports fall, and four areas carry the marks: the gain in producer surplus, the loss in consumer surplus, the tariff revenue collected and the two deadweight triangles. On an exchange rate diagram the vertical axis is the rate and the horizontal axis the quantity of currency, with higher exports adding to the supply of foreign currency and pushing the domestic currency up.
图表题的时间分配与评估落笔
一张四分图题写完坐标轴、两条曲线、均衡点与一句解释，控制在四分钟以内比较合适。Edexcel 经济学 A 的 25 分题按 16 分知识、应用与分析加 9 分评估给分（Pearson 评估支持页），先画图再写分析能省下反复修改的时间。评估段落写清结论依赖的条件，比再重复一遍分析有用。常见的错误是把评估写成第二段分析，始终没有给出判断。剩下两分钟回头检查标签与单位，通常能捡回一分。卷面上留白处画小图，正文里只写结论与条件，读卷的人更容易跟上思路。
Four minutes is a reasonable budget for a four-mark diagram: axes, both curves, the equilibrium and one explanatory sentence. Edexcel Economics A marks its 25-mark questions as 16 marks for knowledge, application and analysis plus 9 for evaluation according to Pearson's assessment support material, so drawing first and writing second saves rewriting time. An evaluation paragraph earns more when it states the conditions the conclusion depends on rather than restating the analysis, and the last two minutes are best spent checking labels and units.
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