需求弹性2
我们今天继续讲需求弹性理论的第二节。
我们先讲定义，再来解释。（大家不用着急也不要被公式和概念给吓住了。）
需求弹性是指，商品的需求量对自身价格的一种反应，或者叫敏感度。用公司表示就是，弹性系数等于，需求量变化的百分比，除以价格变化的百分比。即，得塔Q比得塔P。
我们想合理的估算价格。所以我们想通过这个公式，来考查商品价格的变化，与人们购买量也就是需要量变化的关系。根据常识，并且根据需求曲线我们知道，一般情况下，随着价格提高，即正向变化，人们的购买量也就是需求量是降低的，即反向变化。所以弹性系数一般是负数。
那么有没有例外呢？也有的，吉芬商品的弹性系数就是正的。也就是价格越高，反而买的人越多，需求量越大。（我们今天不讲这个，大家知道有这么回事就行。）
好，我们回到弹性系数。我们为了简便起见，变化的百分比都用绝对值即正数来表示。
举个例子来帮助理解弹性公式。假如说，我现在把商品的价格提高一块钱，那么价格变化算是高还是低呢？ 聪明如你们，当然会说，那要看我们讨论的是什么商品了。
是的，假如我们说的是绿箭口香糖（不小心暴露年龄惹，现在的小朋友可能不知道啥是绿箭。估计更不可能知道啥是大大了。），口香糖的原价就是1块钱，现在我们又提高了1块钱，那价格的变化就是100%。
但如果我们是回到上节课卖儿童读物的那个例子，假如一本书原先是卖100块钱，我们提高1块钱，现在变成101，那增幅就只有1%。
所以当我们讨论定价理论的时候，不仅要思考增加或者打折多少钱，还要看商品之前的原本价格是多少。你看同样是1块钱，对口香糖销量的影响，肯定是远远大于对书的销量的影响的。
我们再用原文例子来学习怎么应用这个公式。作者举了一个克林顿时代提高香烟税的例子。（作者也暴露年龄了，也该把课件更新到特朗普时代）： 如果香烟的价格从100块提高到110块，即增加10%，那么10个年轻人中将会有7个孩子不再购买香烟，即减少70%（-70%）。 套用这个公式，70%除以10%，也就是说，香烟税的弹性系数是-7。
注意到作者的计算错了，但是没关系，我们理解这个弹性理论的公式和概念就好。
原文如下：
Elasticity of Demand 2
来源: 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?
