Chapter 3 Elasticity Theory1 1微观经济理论微观经济理论3Elasticitytheory3ElasticitytheoryThe contents of the chapter 1.What is elasticity? 2. Elasticity of Demand 3. Elasticity of Supply2 2微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory1.What is elasticity?弹性弹性elasticityelasticity： 自变量变动一个百分比所引起的因变量变动自变量变动一个百分比所引起的因变量变动的百分比的百分比。( (注意：不是因变量变动量与自变量变注意：不是因变量变动量与自变量变动量之比。动量之比。) ) e= e= （ Y/Y Y/Y）/ /（ X/X X/X）= =（Y/ XY/ X） （ X/Y X/Y ） 弹性是一个具体的数值弹性是一个具体的数值coefficientcoefficient，与变量之间，与变量之间的度量无关。的度量无关。为什么用弹性：为什么用弹性： 通过弹性，可以表示存在函数关系的变量与变量通过弹性，可以表示存在函数关系的变量与变量之间反应的敏感程度并做出量化分析，从而能更之间反应的敏感程度并做出量化分析，从而能更好地认识变量之间的相互关系。好地认识变量之间的相互关系。3 3微观经济理论微观经济理论3Elasticitytheory3ElasticitytheoryElasticity is a measure of responsiveness. Two words are important here. The word measure means that elasticity results are reported as numbers, or elasticity coefficients. The word responsiveness means that there is a stimulus-reaction involved. Some change or stimulus causes people to react by changing their behavior, and elasticity measures the extent to which people react. 4 4微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory2. Elasticity of Demand Qd=f( P, I, Pi,Pe) 5 5微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory2.1 Price elasticity of demandThe most common elasticity measurement is that of price elasticity of demand. It measures how much consumers respond in their buying decisions to a change in price.6 6微观经济理论微观经济理论3Elasticitytheory3ElasticitytheoryThe basic formula used to determine price elasticity is: 7 7微观经济理论微观经济理论3Elasticitytheory3ElasticitytheoryE.G. If price increases by 10% and consumers respond by decreasing purchases by 20%, the equation computes the elasticity coefficient as -2. The result is negative because an increase in price (a positive number) leads to a decrease in purchases (a negative number). Because the law of demand says it will always be negative, many economists ignore the negative sign.8 8微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory弹性公式Elasticity formula弧弹性公式arc elasticity formula: Q PaEd=- (a (a点到点到b b点的弹性点的弹性) ) P Qa Q PbEd=- (b (b点到点到a a点的弹性点的弹性) ) P Qb9 9微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory弧弹性公式(中点公式) Q/Q/（Q Qa a+Q+Qb b）/2 Q /2 Q P Pa a+P+Pb b Ed = Ed = - - 或或 = =- - P/P/（P Pa a+P+Pb b）/2 P /2 P Q Qa a+Q+Qb b点弹性公式与计算 dQdQ/Q /Q dQdQ P P Ed = Ed =- - 或或 = = - - dPdP/P /P dPdP Q Q1010微观经济理论微观经济理论3Elasticitytheory3ElasticitytheoryE.G.某杂志价格为2元时销售量为5万册，价格为3元时销售量为3万册，则需求价格弹性为多少？解：价格从2元上涨至3元，Ed= 0.8 价格从3元下降至2元，Ed= 2 利用中点公式计算则有Ed=1.251111微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory需求函数：需求函数：Qd =a-bP (aQd =a-bP (a、b b为常数，为常数，b0)b0)，设，设P=1P=1，求点弹性。，求点弹性。 解：P=1，则Qd = a - b 1，另dQ/dP= -b dQ P 1 b Ed = - = b = dP Q a b a b1212微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory需求的价格弧弹性分类The classification of price elasticity of demandThe classification of price elasticity of demand Ed=0，需求完全无弹性Perfectly inelasticPerfectly inelastic 0|Ed|1，需求缺乏弹性inelasticinelastic |Ed|=1，需求具有单位弹性unit elasticelastic 1|Ed|1中点C： |Ed|=1线段CB： |Ed|1, elastic，so dTR/dP0, P TREd0, P TR Ed=1,unit elastic， dTR/dP=0,no affectEd=0，perfectly inelastic收入同比例于价格变化而变化。Ed=，perfectly elastic既定价格下收益可无限增加，厂商不会降价，涨价则会使收入减少为2020微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory影响需求价格弹性的因素The factors that affect the price elasticity of demandThe factors that affect the price elasticity of demand商品的可替代性商品的可替代性 Substitutability of goods Substitutability of goods商品用途的广泛性商品用途的广泛性 Extensive use of goods Extensive use of goods商品对消费者的重要程度商品对消费者的重要程度(preferrence)(preferrence) Importance of goods to consumers Importance of goods to consumers商品的消费支出在消费总支出中所占的比重商品的消费支出在消费总支出中所占的比重 The proportion of consumer spending in the consumer goods The proportion of consumer spending in the consumer goods in total expenditurein total expenditure消费者调节需求量的时间消费者调节需求量的时间 Time for consumer demand Time for consumer demand 2121微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory需求收入弹性 Income elasticity of demandIt measures how the quantity demanded changes as consumer income changes.It is calculated as: Percentage change in quantity demandedEi= Percentage change in income2222微观经济理论微观经济理论3Elasticitytheory3Elasticitytheoryformula： Q/Q Q I Ei = 或 = I/I I QClassify： Ey0,Normal, good; Ey0，inferior good 0 Ey1， Luxury2323微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory需求交叉弹性Cross-price elasticity of demandDefinition：It measures how the quantity demanded of one good changes as the price of another good changes.It is calculated as : Percentage change in quantity of good xPercentage change in quantity of good x Exy= Percentage change in the price of good yPercentage change in the price of good y2424微观经济理论微观经济理论3Elasticitytheory3ElasticitytheoryFormula： Qy/Qy Qy Px Exy = or = Px/Px Px QyClassification： Exy0，Substitute 替代品； Exy0)d0)，P=1P=1，then the point Es will then the point Es will be ?be ? P=1，Qs = c+d，dQ/dP= d dQ P 1 d Es = = d = dP Q c+d c+d3131微观经济理论微观经济理论3Elasticitytheory3ElasticitytheoryClassification Es=0，供给完全无弹性perfectly inelastic0Es1，供给缺乏弹性inelasticEs=1，供给具有单位弹性unit elastic1Es，供给富有弹性elasticEs=，供给有无限弹性 perfectly elastic3232微观经济理论微观经济理论3Elasticitytheory3ElasticitytheoryClassification for EsOPQS1 Es=0S5 Es=S3 Es=1S2 Es13333微观经济理论微观经济理论3Elasticitytheory3ElasticitytheoryThe factors affecting EsTime for producer adjusting yeildMarginal costProduction cycle3434微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory4.The applications of elasticity theory谷贱伤农 The lower price of food will reduce the revenue of farmers税负分担理论 The theory of tax burden蛛网理论 Spider-web theory3535微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory谷贱伤农QopE1P1Q1E2P2Q2FDS1S23636微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory税负分担理论oQPS0E0P0Q0D0S1E1P1Q1P2tD0E1P1Q1P2弹性不同税收的分担也不同3737微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory蛛网模型基本假设： (1)本期的供给量取决于上期的价格； (2)本期的需求量取决于本期的价格。三种类型：收敛型蛛网、发散型蛛网、稳定型蛛网适用分析对象：有生产周期的产品。3838微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory蛛网模型（数学方法）1.Qdt=Qst 2.Qdt=a-bPt (a,b0)3.Qst=-c+dPt-1 (c,d0)由1、2、3可得：bPt + dPt-1 +=a+c将上式正规化，得Pt +1+（b/d）Pt=（a+c）/b3939微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory蛛网模型（数学方法）解上差分方程：Pt=P0-(a+c)/(b+d)(-d/b)t+ (a+c)/(b+d)P0表示初始价格在市场均衡时， Pe = Pt = Pt +1因此有： Pe= (a+c)/(b+d)，将Pe代入解中，Pt=（P0 Pe ） (-d/b)t+ Pe4040微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory蛛网模型（数学方法）通过上式子，可以看出：1.当db时，震荡将为放大震荡2.当d=b时，震荡为单位震荡3.当d0)Qst=-c+dPt-1 Pt-1 =c/d+(1/d) Qst (c,d0)4141微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory收敛型蛛网（图示）oQPEDSP*Q*P0Q1P1Q2P2Q3P3Q44242微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory发散型蛛网（图示）oQPEDSP*Q*P0Q1P1Q2P2Q3P3Q44343微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory稳定型蛛网（图示）oQPDSEP*Q*P0Q1P0P1Q2P2Q3P3Q44444微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory三类蛛网模型的条件收敛型蛛网的条件：供给弹性需求曲线斜率（绝对值）。发散型蛛网的条件：供给弹性需求弹性，或，供给曲线斜率需求曲线斜率（绝对值）。稳定型蛛网的条件：供给弹性=需求弹性，或，供给曲线斜率=需求曲线斜率（绝对值）。4545微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory思考题：相对而言，为什么化妆品可以薄利多销而药品不可以？从经济学角度，你认为怎样可以减少迷幻商品(指烟、酒、毒品等)？4646微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory4747微观经济理论微观经济理论3Elasticitytheory3Elasticitytheory