A-level economics revision guides and question banks covering labour markets, supply and demand, market structure and all core economics a-level topics.
以下内容我们会持续更新：
今天带给大家Alevel经济笔记是：
第三章：Elasticities—
Income Elasticity of Demand Formulae
Income Elasticity of Demand
Income Elasticity of Demand FormulaeA definition and the formula定义和公式
With the price elasticity of demand, we were measuring the responsiveness of demand to a change in price.
通过需求的价格弹性，我们测量了需求对价格变化的反应。
With the cross price elasticity of demand, we were measuring the responsiveness of the demand for one good with respect to a change in price of another good.
通过需求的交叉价格弹性，我们测量了一种商品的需求对另一种商品价格变化的反应。
With the price elasticity of supply, we were measuring the responsiveness of supply to a change in price.
通过供给的价格弹性，我们测量了供给对价格变化的反应。
With the income elasticity of demand, one measures the responsiveness of demand to a change in consumers' real income.
使用需求的收入弹性，可以衡量需求对消费者实际收入变化的反应程度。
The following formula can be used to measure exactly how responsive demand is to a given change in income:
以下公式可用于准确衡量需求对给定收入变化的响应程度：
Where:
EY = The income elasticity of demand
Δ = 'change in'
Qd = Quantity demanded
Y = Real income
Using the formulaThe structure of this formula is exactly the same as for the other three elasticities.
If you need a more detailed explanation of how to use this type of formula, go back to the Learn-It on 'the price elasticity of demand'.
Otherwise, try these three questions. Once you have done them, click the appropriate button to get the answer and the required working.
If you got one or more of the questions wrong, check the working carefully to make sure that you understand where you went wrong.
该公式的结构与其他三个弹性完全相同。如果您需要有关如何使用此类公式的更详细说明，请返回“需求价格弹性”的 Learn-It。否则，试试这三个问题。完成后，单击相应的按钮以获取答案和所需的工作。如果你做错了一个或多个问题，请仔细检查工作，以确保你明白哪里出错了。
Average real incomes rise by 8% over a given year. As a result, the demand for cars rises by 24%. What is the income elasticity of demand (ceteris paribus)?
平均实际收入在某一年增加了 8%。因此，对汽车的需求增加了 24%。需求的收入弹性是多少（其他条件不变）？
Average real incomes rise by 5% over a given year. As a result, the demand for bus travel falls by 10%. What is the income elasticity of demand (ceteris paribus)?
平均实际收入在某一年增加了 5%。因此，对公共汽车旅行的需求下降了 10%。需求的收入弹性是多少（其他条件不变）？
During a recession, average real incomes fall by 5% over a given year. The income elasticity of demand with respect to Brand X lageris 2.5. If initial sales of Brand X nationwide were 25 million litres, what will the new sales figure be as a result of the rise in income (ceteris paribus)?
在经济衰退期间，特定年份的平均实际收入下降了 5%。品牌 X lageris 的需求收入弹性 2.5。如果 X 品牌在全国的初始销量为 2500 万升，由于收入增加（其他条件不变），新的销量数字将是多少？
Normal and inferior goods
正常商品和劣质商品
In the questions you tried above, notice that the value for the income elasticity of demand can be positive or negative, a bit like the cross price elasticity of demand.
在您上面尝试的问题中，请注意需求的收入弹性值可以是正值或负值，有点像需求的交叉价格弹性。
Most goods are normal goods.
A normal good is one where, as one would expect, its demand rises as consumers' income rises.
There is a positive relationship between real income and the demand for the good in question. Of course, this also means that the demand for these types of goods falls when real incomes fall, as the third question above demonstrated.
大多数商品都是普通商品。正如人们所预料的那样，普通商品是一种需求随着消费者收入的增加而增加的商品。实际收入与相关商品的需求之间存在正相关关系。当然，这也意味着当实际收入下降时，对这类商品的需求也会下降，正如上面第三个问题所表明的那样。
Some goods are known as inferior goods.
With inferior goods, there is an inverse relationship between real income and the demand for the good in question.
If real incomes rise, the demand for an inferior good will fall.
If real incomes fall (in a recession, for instance), the demand for an inferior good will rise. Here are a couple of classic examples of inferior goods:
有些商品被称为劣质商品。对于劣质商品，实际收入与对相关商品的需求之间存在反比关系。如果实际收入增加，对劣质商品的需求就会下降。如果实际收入下降（例如在经济衰退中），对劣质商品的需求将会上升。以下是劣质商品的几个经典例子：
Black and white TVs.
This one is a bit old fashioned.
The idea is that as people get richer, they are more likely to buy a new colour TV than an inferior black and white TV.
A slightly more up-to-date example might be audiotapes, as more people buy CD players.
黑白电视。这个有点老土了。这个想法是，随着人们变得更富有，他们更有可能购买新的彩色电视，而不是劣质的黑白电视。一个稍微更新的例子可能是录音带，因为越来越多的人购买 CD 播放器。
Bus travel.
Notice that the second question above referred to this example.
As people get richer, they are more likely to buy themselves a car, or use a taxi, rather than rely on the more inferior bus, so the demand for bus travel falls as real incomes rise.
巴士旅行。请注意，上面的第二个问题提到了这个例子。随着人们变得更富有，他们更有可能给自己买车或乘坐出租车，而不是依赖更劣质的公共汽车，因此随着实际收入的增加，对公共汽车旅行的需求下降。
The values of income elasticity of demand for normal and inferior goods
正常商品和劣质商品需求的收入弹性值
Given the analysis above, you have probably worked out that normal goods are ones with a positive income elasticity of demand, whereas inferior goods have a negative income elasticity of demand.
根据上面的分析，您可能已经计算出正常商品的需求收入弹性为正，而劣质商品的需求收入弹性为负。
Some text books sub-divide normal goods into necessities and luxury goods.
一些教科书将普通商品细分为必需品和奢侈品。
Necessities are defined as goods that are normal (so EY > 0), but the demand for these goods does not rise that much as a result of an increase in real incomes.
If one's real income doubles overnight, would one really buy twice as much bread?
Probably not. Perhaps four loaves a week instead of three so that one can throw out half a loaf mid-week and start a fresh one. Bread is a necessity, not a luxury.
For a given percentage rise in real incomes, the demand for these goods tends to rise, but by a smaller percentage.
So the value for the income elasticity of demand is between zero and one.
必需品被定义为正常的商品（因此 E Y > 0），但由于实际收入的增加，对这些商品的需求并没有增加那么多。如果一个人的实际收入在一夜之间翻了一番，真的会买两倍的面包吗？可能不是。也许一周吃四个面包而不是三个，这样一个人就可以在周中扔掉半个面包并开始一个新的。面包是必需品，不是奢侈品。对于实际收入的特定百分比增长，对这些商品的需求往往会上升，但上升的百分比较小。因此，需求的收入弹性值介于 0 和 1 之间。
Luxuries, therefore, are goods whose income elasticity of demand is greater than one.
If real incomes in the UK rise quite significantly, the demand for things like fast cars, expensive holidays abroad and dishwashers tend to rise at an even faster rate.
Many people who did not purchase these goods and services at all will now have enough money to get involved.
因此，奢侈品是需求收入弹性大于 1 的商品。如果英国的实际收入显着增加，对快速汽车、昂贵的国外假期和洗碗机等物品的需求往往会以更快的速度增长。许多根本没有购买这些商品和服务的人现在将有足够的钱参与其中。
The table below summarises the above:下表总结了上述内容：
|
Normal or inferior good? |
Necessity or luxury? |
Examples |
Value of the income elasticity of demand |
|
Normal |
Necessity |
Basic foodstuffs, including bread, tea, fruit and green vegetables. |
0 < EY < 1 |
|
Luxury |
Foreign holidays, fast cars, meals out and fine wines. |
EY > 1 | |
|
Inferior |
Bus travel and audio tapes |
EY < 0 |
Income and substitution effects收入和替代效应
Before we move on to explain what a 'giffen' good is, it is important that you understand how the income effect and the substitution effect of a price change affect the quantity demanded of different types of goods.
在我们继续解释什么是“吉芬”商品之前，重要的是要了解价格变化的收入效应和替代效应如何影响不同类型商品的需求量。
The substitution effect of a price change价格变动的替代效应
This is the easy one.
For all goods, a rise in price relative to other goods will cause consumers to demand less of the good in question and more of the other goods.
If the price of a good fell, ceteris paribus, then consumers would buy more of that good at the expense of other goods that are now relatively dear.
Consumers substitute purchases of the relatively more expensive good for the relatively cheaper good following the price change.
这是最容易的。对于所有商品，相对于其他商品的价格上涨将导致消费者对相关商品的需求减少，而对其他商品的需求增加。如果一种商品的价格下降，在其他条件不变的情况下，消费者会以牺牲现在相对昂贵的其他商品为代价购买更多这种商品。随着价格的变化，消费者用相对便宜的商品替代了相对较贵的商品的购买。
The income effect of a price change价格变动的收入效应
This one is a bit harder. When the price of a good rises, ceteris paribus, then a consumer's real income will fall.
这个有点难。在其他条件不变的情况下，当一种商品的价格上涨时，消费者的实际收入就会下降。
Imagine that you earned £100 a week, after tax, and you spent this money on only two types of good: you buy 6 CDs at £10 a CD and visit a club four times at £10 a go.
Now assume that the price of CDs rose to £15. Next week, if you still want to go clubbing four times, you will only be able to buy 4 CDs.
The amount that you can really buy has fallen given your fixed nominal income of £100. Your real income has fallen.
想象一下，税后您每周赚 100 英镑，而您只将这笔钱花在了两种商品上：您以每张 10 英镑的价格购买了 6 张 CD，并以每次 10 英镑的价格参观了俱乐部四次。现在假设 CD 的价格上涨到 15 英镑。下周，如果你还想去夜总会四次，你将只能购买 4 张 CD。鉴于您的固定名义收入为 100 英镑，您真正可以购买的金额已经下降。你的实际收入下降了。
The effect on the demand for various goods following a price rise as a result of the income effect will depend on whether the good in question is normal or inferior.
Following the price rise, real incomes fall, so for a normal good the income effect will cause the demand for the good to fall (because of the positive income elasticity).
For an inferior good, the fall in real income will lead to a rise in the demand for the good as a result of the income effect (because of the negative income elasticity).
收入效应导致价格上涨后对各种商品需求的影响将取决于所讨论的商品是正常的还是劣质的。随着价格上涨，实际收入下降，因此对于正常商品，收入效应将导致对该商品的需求下降（因为收入弹性为正）。对于劣质商品，由于收入效应（由于负收入弹性） ，实际收入的下降将导致对该商品的需求增加。
Combining the income and substitution effects结合收入和替代效应
The table below summarises what we have said so far.
The effects below assume that there has been a rise in the price of the good in question.
下表总结了我们到目前为止所说的内容。下面的影响假设相关商品的价格已经上涨。
|
Type of good |
The effect on quantity demanded due to the Substitution effect |
The effect on quantity demanded due to the Income effect |
The Total effect on quantity demanded |
|
Normal good |
Fall |
Fall |
Fall |
|
Inferior good |
Fall |
Rise |
Fall (because the substitution effect is thought to be stronger than the income effect) |
In other words, If the price of a normal good rises then its demand will fall, as one would expect, resulting in a downward sloping demand curve.
If the price of an inferior good rises then its demand will fall, but perhaps not as much as with a normal good.
But still, its demand curve will be downward sloping, as expected.
The above analysis would be equal and opposite if we considered a price fall.
换句话说，如果一种正常商品的价格上涨，那么它的需求就会下降，正如人们所预料的那样，导致需求曲线向下倾斜。如果劣质商品的价格上涨，那么它的需求就会下降，但可能不会像普通商品那样下降。但是，正如预期的那样，其需求曲线将向下倾斜。如果我们考虑价格下跌，上述分析将是相同和相反的。
Giffen goods吉芬商品
The Giffen good is an unusual type of inferior good. Basically, it is a very inferior good! I think the best example is potatoes during the potato famine in Ireland.
吉芬商品是一种不寻常的劣质商品。基本上是很劣质的好东西！我认为最好的例子是爱尔兰马铃薯饥荒期间的马铃薯。
During the potato famine of the nineteenth century, appalling harvests meant that the price of potatoes rose (you might want to draw a supply and demand curve to explain this!).
If it was a normal, or even an inferior good, one would have expected the demand for potatoes to fall as a result of this price rise.
What actually happened was that the demand for potatoes rose?!?
在 19 世纪的马铃薯饥荒期间，惊人的收成意味着马铃薯的价格上涨（你可能想画一条供求曲线来解释这一点！）。如果它是一种正常的，甚至是劣质的商品，人们会预期土豆的需求会因为价格上涨而下降。究竟发生了什么，土豆的需求上升了？！？
Why did this happen?
The key point to understand is that potatoes formed a huge part of a poor Irishman's diet in those desperate days.
People spent most of their income on potatoes, leaving a small proportion of their income to purchase, say, meat; a source of protein and a relative luxury.
When the price of potatoes rose sharply, the income effect of this price change was immense.
Real incomes fell dramatically. The reaction of the Irish was sensible, in a way.
They decided to stop buying the relative luxury, meat, all together, as their real incomes had fallen so much, and spend all of their income on the one good without which they could not survive: potatoes.
So the result of the large rise in the price of potatoes was an increase in demand for potatoes!
The income effect was much stronger than the substitution effect and, unusually, the demand curve for potatoes was upward sloping!
为什么会这样？要理解的关键点是，在那些绝望的日子里，土豆是一个贫穷的爱尔兰人饮食的重要组成部分。人们将大部分收入花在土豆上，只剩下一小部分收入用于购买肉类等；蛋白质的来源和相对的奢侈品。当土豆价格大幅上涨时，这种价格变化带来的收入效应是巨大的。实际收入急剧下降。在某种程度上，爱尔兰人的反应是明智的。他们决定停止购买相对奢侈的肉类，因为他们的实际收入已经大幅下降，而将所有收入都花在了一种他们无法生存的商品上：土豆。因此，马铃薯价格大幅上涨的结果是增加了对土豆的需求！收入效应比替代效应强得多，而且不同寻常的是，马铃薯的需求曲线是向上倾斜的！
The table below summarises the situation following the large rise in the price of the giffen good:下表总结了吉芬商品价格大幅上涨后的情况：
|
Type of good |
The effect on quantity demanded due to the Substitution effect |
The effect on quantity demanded due to the Income effect |
The Total effect on quantity demanded |
|
Giffen good |
Fall |
Rise |
Rise (because the income effect was much stronger than the substitution effect) |
There are a few other examples of goods that have (unusually) upward sloping demand curves.
Very expensive goods with a 'keeping up with the Jones' factor, for instance.
If the price of a Rolls Royce rises, their demand can, perversely, rise, as more people want to be 'seen' in the new, more expensive model! It has 'snob' appeal.
The best example is probably the stock market boom leading up to the crash in October 1987. Speculation of never ending price rises caused demand to keep rising.
A price rise was seen as a 'good thing' causing further increases in demand, even if the economic fundamentals of the company whose stock was popular were unchanged.
Remember, though, that example of goods with upward sloping demand curves are few and far between. The downward sloping curve is true of nearly all goods and services.
还有其他一些具有（异常）向上倾斜的需求曲线的商品示例。例如，具有“跟上琼斯”因素的非常昂贵的商品。如果劳斯莱斯的价格上涨，他们的需求就会反常地上升，因为越来越多的人希望在新的、更昂贵的车型中被“看到”！它具有“势利”的吸引力。最好的例子可能是导致 1987 年 10 月崩盘的股市繁荣。对价格永无止境的猜测导致需求持续上升。即使股票受欢迎的公司的经济基本面没有变化，价格上涨也被视为导致需求进一步增加的“好事”。但是请记住，具有向上倾斜的需求曲线的商品示例很少而且相差甚远。向下倾斜的曲线几乎适用于所有商品和服务。
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关于Alevel经济笔记第三章：Elasticities后续的更新计划如下：
