需求彈性2
我們今天繼續講需求彈性理論的第二節。
我們先講定義，再來解釋。（大家不用著急也不要被公式和概念給嚇住了。）
需求彈性是指，商品的需求量對自身價格的一種反應，或者叫敏感度。用公司表示就是，彈性系數等于，需求量變化的百分比，除以價格變化的百分比。即，得塔Q比得塔P。
我們想合理的估算價格。所以我們想通過這個公式，來考查商品價格的變化，與人們購買量也就是需要量變化的關系。根據常識，并且根據需求曲線我們知道，一般情況下，隨著價格提高，即正向變化，人們的購買量也就是需求量是降低的，即反向變化。所以彈性系數一般是負數。
那么有沒有例外呢？也有的，吉芬商品的彈性系數就是正的。也就是價格越高，反而買的人越多，需求量越大。（我們今天不講這個，大家知道有這么回事就行。）
好，我們回到彈性系數。我們為了簡便起見，變化的百分比都用絕對值即正數來表示。
舉個例子來幫助理解彈性公式。假如說，我現在把商品的價格提高一塊錢，那么價格變化算是高還是低呢？ 聰明如你們，當然會說，那要看我們討論的是什么商品了。
是的，假如我們說的是綠箭口香糖（不小心暴露年齡惹，現在的小朋友可能不知道啥是綠箭。估計更不可能知道啥是大大了。），口香糖的原價就是1塊錢，現在我們又提高了1塊錢，那價格的變化就是100%。
但如果我們是回到上節課賣兒童讀物的那個例子，假如一本書原先是賣100塊錢，我們提高1塊錢，現在變成101，那增幅就只有1%。
所以當我們討論定價理論的時候，不僅要思考增加或者打折多少錢，還要看商品之前的原本價格是多少。你看同樣是1塊錢，對口香糖銷量的影響，肯定是遠遠大于對書的銷量的影響的。
我們再用原文例子來學習怎么應用這個公式。作者舉了一個克林頓時代提高香煙稅的例子。（作者也暴露年齡了，也該把課件更新到特朗普時代）： 如果香煙的價格從100塊提高到110塊，即增加10%，那么10個年輕人中將會有7個孩子不再購買香煙，即減少70%（-70%）。 套用這個公式，70%除以10%，也就是說，香煙稅的彈性系數是-7。
注意到作者的計算錯了，但是沒關系，我們理解這個彈性理論的公式和概念就好。
原文如下：
Elasticity of Demand 2
來源: 作者Dr. Mary J. McGlasson
Elasticity is a measure of sensitivity, or responsiveness, to price.
In equation form, the elasticity of demand, or ed, is equal to the percentage change in quantity demanded over the percentage change in price.
Because demand exhibits an inverse, or negative, relationship, elasticity of demand will be a negative number.
I use percentage change to measure elasticity, rather than absolute change -- let me illustrate why.
If I tell you that product price has gone up by one dollar, this would be the "absolute change."
Is this a big change, or a small change?
It depends -- what's the product?
More to the point, what was the original price?
OK, look – say we're talking about a pack of gum.
Originally the price was one dollar; now it's two dollars.
This represents an absolute change of one dollar, but is it a big change, or a small change?
It's actually a pretty big change; price doubled, or increased by 100%.
What if we're talking about a textbook, rather than a pack of gum?
Originally, the price was $100; now it's $101.
This is still an absolute change of one dollar, but is it a big change, or a small change?
In this case, it's a small change; prices increased by 1%.
Bottom line is that we need to know not only the dollar amount of the price change, but also how this compares to where we started.
Now technically, the formula for elasticity of demand is the percentage change in the quantity demanded over the percentage change in price,
which can be found by taking the ratio of the difference between the new and the old quantities,
over the average of the new the old quantities, all over the ratio of the difference between the new and the old price, over that the average of the new and the old prices...
Frankly, I've found that if I use this version of the elasticity formula, students' eyes glaze over.
People get so hung up on the math that they lose sight of the intuition, and what elasticity means --
so I'll be sticking to the slightly easier form, and will frame my questions for you accordingly.
How would you actually use this formula?
Take a look at this article about the Clinton administration's proposed cigarette tax policy.
If you look at the last paragraph, you'll find enough information to determine the elasticity of demand for youth smoking.
Remember, elasticity of demand is the percentage change in quantity demanded, over the percentage change in price.
The article states that for every 10% increase in price, there's a 7% decrease in youth smoking.
This means that elasticity of demand, according to the formula, is -7% over +10%, or -.7.
OK -- now what do I do?
I know that elasticity of demand for youth smoking is -.7, but what does it mean?
