【A-Level經(jīng)濟(jì)精講】需求的收入彈性（Income Elasticity of Demand）

今天要給大家講解的是Elasticity of demand中的最后一個(gè)Elasticity：Income elasticity of demand，它所包含的知識(shí)點(diǎn)也比較細(xì)，所以建議同學(xué)們?cè)谧屑?xì)閱讀后分類(lèi)記憶，同時(shí)也可以跟之前學(xué)習(xí)的PED以及XED進(jìn)行比較和聯(lián)系，Elasticity這一塊的知識(shí)點(diǎn)都有些零碎，也要注意定時(shí)復(fù)習(xí)回顧哦。

Income elasticity of demand的定義：

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

計(jì)算公式:

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

YED value分為兩種情況來(lái)討論，一種是Normal good，一種是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.當(dāng)收入增加，人們會(huì)購(gòu)買(mǎi)更多的正常商品，

當(dāng)收入減少，人們會(huì)購(gòu)買(mǎi)更少的正常商品。

所以，Normal good的YED value一定是正數(shù)。

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.
當(dāng)收入增加，人們會(huì)購(gòu)買(mǎi)更少的劣質(zhì)品，當(dāng)收入減少，人們會(huì)購(gòu)買(mǎi)更多的劣質(zhì)品。

所以，Inferior good的YED value一定是負(fù)數(shù)。

綜上所述，YED的Mathematical sign可以告訴我們Product是Normal good還是Inferior good。

同時(shí)，根據(jù)YED value的絕對(duì)值大小，又可以將其分成四類(lèi)。（以下數(shù)字全部為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數(shù)值的絕對(duì)值告訴了我們一個(gè)產(chǎn)品對(duì)于收入是否敏感。

YED的用途

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

如何決定Production level需要借助Economic cycle，如下圖：

當(dāng)處于Growth/boom的時(shí)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.

增加正常商品的生產(chǎn)，減少劣質(zhì)品的生產(chǎn)。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.

增加劣質(zhì)品的生產(chǎn)，減少正常商品的生產(chǎn)。