【A-Level经济精讲】需求的收入弹性（Income Elasticity of Demand）

今天要给大家讲解的是Elasticity of demand中的最后一个Elasticity：Income elasticity of demand，它所包含的知识点也比较细，所以建议同学们在仔细阅读后分类记忆，同时也可以跟之前学习的PED以及XED进行比较和联系，Elasticity这一块的知识点都有些零碎，也要注意定时复习回顾哦。

Income elasticity of demand的定义：

It measures the responsiveness of quantity demanded when there is a change in income.

计算公式:

% of change in quantity demanded/% of change in income.

YED value分为两种情况来讨论，一种是Normal good，一种是Inferior good。Normal goodFor normal good, when the income of people is increasing, the quantity demanded for normal good will increase; When the income of people is decreasing, the quantity demanded for normal good will decrease.当收入增加，人们会购买更多的正常商品，

当收入减少，人们会购买更少的正常商品。

所以，Normal good的YED value一定是正数。

Inferior goodFor inferior good, when the income of people is increasing, the quantity demanded for inferior good will decrease; When the income of people is decreasing, the quantity demanded for inferior good will increase.
当收入增加，人们会购买更少的劣质品，当收入减少，人们会购买更多的劣质品。

所以，Inferior good的YED value一定是负数。

综上所述，YED的Mathematical sign可以告诉我们Product是Normal good还是Inferior good。

同时，根据YED value的绝对值大小，又可以将其分成四类。（以下数字全部为assumption）1YED=3 (Income elastic)The product is normal good, e.g.luxury bags2YED=0.6 (Income inelastic)The product is normal good, e.g.necessities3 YED=-2.1 (Income elastic)The product is inferior good, e.g.frozen dinner4YED=-0.2 (Income inelastic)The product is inferior good, e.g.canned fruits

The extent(Absolute value) of YED tells us whether or not the QD of a product is sensitive to income change.

YED数值的绝对值告诉了我们一个产品对于收入是否敏感。

YED的用途

It can helps to decide the production level and nature of a firm.

如何决定Production level需要借助Economic cycle，如下图：

当处于Growth/boom的时候Should increase the production of positive YED products, which are normal goods, luxury, necessities and decrease the production of negative YED products, which are inferior goods.

增加正常商品的生产，减少劣质品的生产。Recession/slumpShould decrease the production of positive YED products, which are normal goods, luxury, necessities and increase the production of negative YED products, which are inferior goods.

增加劣质品的生产，减少正常商品的生产。