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Chapter 7 MARKET DEMAND AND ELASTICITY Copyright ©2002 by South-Western, a division of Thomson Learning. All rights reserved. MICROECONOMIC THEORY BASIC PRINCIPLES AND EXTENSIONS EIGHTH EDITION WALTER NICHOLSON

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Market Demand Curves Assume that there are only two goods (X and Y) and two individuals (1 and 2) –The first person’s demand for X is X 1 = d X 1 (P X,P Y, I 1 ) –The second person’s demand for X is X 2 = d X 2 (P X,P Y, I 2 )

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Market Demand Curves Features of these demand curves: –Both individuals are assumed to face the same prices –Each buyer is assumed to be a price taker must accept the prices prevailing in the market –Each person’s demand depends on his or her own income

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Market Demand Curves The total demand for X is the sum of the amounts demanded by the two buyers –The demand function will depend on P X, P Y, I 1, and I 2 total X = X 1 + X 2 total X = d X 1 (P X,P Y, I 1 ) + d X 2 (P X,P Y, I 2 ) total X = D X (P X,P Y, I 1, I 2 )

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Market Demand Curves To construct the market demand curve, P X is allowed to vary while P Y, I 1, and I 2 are held constant If each individual’s demand for X is downward sloping, the market demand curve will also be downward sloping

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Market Demand Curves XXX PXPX PXPX PXPX dX1dX1 dX2dX2 X1*X1* X2*X2* PX*PX* To derive the market demand curve, we sum the quantities demanded at every price Individual 1’s demand curve Individual 2’s demand curve Market demand curve X*X* DXDX X 1 * + X 2 * = X*

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Shifts in the Market Demand Curve The market demand summarizes the ceteris paribus relationship between X and P X –Changes in P X result in movements along the curve (change in quantity demanded) –Changes in other determinants of the demand for X cause the demand curve to shift to a new position (change in demand)

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Shifts in Market Demand Suppose that individual 1’s demand for oranges is given by X 1 = 10 – 2P X + 0.1 I 1 + 0.5P Y and individual 2’s demand is X 2 = 17 – P X + 0.05 I 2 + 0.5P Y The market demand curve is X = X 1 + X 2 = 27 – 3P X + 0.1 I 1 + 0.05 I 2 + P Y

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Shifts in Market Demand To graph the demand curve, we must assume values for P Y, I 1, and I 2 If P Y = 4, I 1 = 40, and I 2 = 20, the market demand curve becomes X = 27 – 3P X + 4 + 1 + 4 = 36 – 3P X

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Shifts in Market Demand If P Y rises to 6, the market demand curve shifts outward to X = 27 – 3P X + 4 + 1 + 6 = 38 – 3P X –Note that X and Y are substitutes If I 1 fell to 30 while I 2 rose to 30, the market demand would shift inward to X = 27 – 3P X + 3 + 1.5 + 4 = 35.5 – 3P X –Note that X is a normal good for both buyers

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Generalizations Suppose that there are n goods (X i, i = 1,n) with prices P i, i = 1,n. Assume that there are m individuals in the economy The j th’s demand for the i th good will depend on all prices and on I j X ij = d ij (P 1,…,P n, I j )

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Generalizations The market demand function for X i is the sum of each individual’s demand for that good The market demand function depends on the prices of all goods and the incomes and preferences of all buyers

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Elasticity Suppose that a particular variable (B) depends on another variable (A) B = f(A…) We define the elasticity of B with respect to A as –The elasticity shows how B responds (ceteris paribus) to a 1 percent change in A

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Price Elasticity of Demand The most important elasticity is the price elasticity of demand –measures the change in quantity demanded caused by a change in the price of the good e Q,P will generally be negative –except in cases of Giffen’s paradox

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Distinguishing Values of e Q,P Value of e Q,P at a Point Classification of Elasticity at This Point e Q,P < -1Elastic e Q,P = -1Unit Elastic e Q,P > -1Inelastic

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Price Elasticity and Total Expenditure Total expenditure on any good is equal to total expenditure = PQ Using elasticity, we can determine how total expenditure changes when the price of a good changes

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Price Elasticity and Total Expenditure Differentiating total expenditure with respect to P yields Dividing both sides by Q, we get

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Price Elasticity and Total Expenditure Note that the sign of PQ/ P depends on whether e Q,P is greater or less than -1 –If e Q,P > -1, demand is inelastic and price and total expenditures move in the same direction –If e Q,P < -1, demand is elastic and price and total expenditures move in opposite directions

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Price Elasticity and Total Expenditure Responses of PQ DemandPrice IncreasePrice Decrease ElasticFallsRises Unit ElasticNo Change InelasticRisesFalls

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Income Elasticity of Demand The income elasticity of demand (e Q, I ) measures the relationship between income changes and quantity changes Normal goods e Q, I > 0 –Luxury goods e Q, I > 1 Inferior goods e Q, I < 0

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Cross-Price Elasticity of Demand The cross-price elasticity of demand (e Q,P’ ) measures the relationship between changes in the price of one good and and quantity changes in another Gross substitutes e Q,P’ > 0 Gross complements e Q,P’ < 0

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Relationships Among Elasticities Suppose that there are only two goods (X and Y) so that the budget constraint is given by P X X + P Y Y = I The individual’s demand functions are X = d X (P X,P Y, I ) Y = d Y (P X,P Y, I )

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Relationships Among Elasticities Differentiation of the budget constraint with respect to I yields Multiplying each item by 1

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Relationships Among Elasticities Since (P X · X)/ I is the proportion of income spent on X and (P Y · Y)/ I is the proportion of income spent on Y, s X e X, I + s Y e Y, I = 1 For every good that has an income elasticity of demand less than 1, there must be goods that have income elasticities greater than 1

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Slutsky Equation in Elasticities The Slutsky equation shows how an individual’s demand for a good responds to a change in price Multiplying by P X /X yields

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Slutsky Equation in Elasticities Multiplying the final term by I / I yields

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Slutsky Equation in Elasticities A substitution elasticity shows how the compensated demand for X responds to proportional compensated price changes –it is the price elasticity of demand for movement along the compensated demand curve

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Slutsky Equation in Elasticities Thus, the Slutsky relationship can be shown in elasticity form It shows how the price elasticity of demand can be disaggregated into substitution and income components –Note that the relative size of the income component depends on the proportion of total expenditures devoted to the good (s X )

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Homogeneity Remember that demand functions are homogeneous of degree zero in all prices and income Euler’s theorem for homogenous functions shows that

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Homogeneity Dividing by X, we get Using our definitions, this means that An equal percentage change in all prices and income will leave the quantity of X demanded unchanged

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Cobb-Douglas Elasticities The Cobb-Douglas utility function is U(X,Y) = X Y The demand functions for X and Y are The elasticities can be calculated

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Cobb-Douglas Elasticities Similar calculations show Note that

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Cobb-Douglas Elasticities Homogeneity can be shown for these elasticities The elasticity version of the Slutsky equation can also be used

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Cobb-Douglas Elasticities The price elasticity of demand for this compensated demand function is equal to (minus) the expenditure share of the other good More generally where is the elasticity of substitution

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Linear Demand Q = a + bP + cI + dP’ where: Q = quantity demanded P = price of the good I = income P’ = price of other goods a, b, c, d = various demand parameters

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Linear Demand Q = a + bP + cI + dP’ Assume that: – Q/ P = b 0 (no Giffen’s paradox) – Q/ I = c 0 (the good is a normal good) – Q/ P’ = d ⋛ 0 (depending on whether the other good is a gross substitute or gross complement)

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Linear Demand If I and P’ are held constant at I * and P’*, the demand function can be written Q = a’ + bP where a’ = a + c I * + dP’* –Note that this implies a linear demand curve –Changes in I or P’ will alter a’ and shift the demand curve

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Linear Demand Along a linear demand curve, the slope ( Q/ P) is constant –the price elasticity of demand will not be constant along the demand curve As price rises and quantity falls, the elasticity will become a larger negative number (b < 0)

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Linear Demand Q P -a’/b a’ e Q,P < -1 e Q,P = -1 e Q,P > -1 Demand becomes more elastic at higher prices

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Constant Elasticity Functions If one wanted elasticities that were constant over a range of prices, this demand function can be used Q = aP b I c P’ d where a > 0, b 0, c 0, and d ⋛ 0. For particular values of I and P’, Q = a’P b where a’ = a I c P’ d

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Constant Elasticity Functions This equation can also be written as ln Q = ln a’ + b ln P Applying the definition of elasticity, The price elasticity of demand is equal to the exponent on P

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Important Points to Note: The market demand curve is negatively sloped on the assumption that most individuals will buy more of a good when the price falls –it is assumed that Giffen’s paradox does not occur Effects of movements along the demand curve are measured by the price elasticity of demand (e Q,P ) –% change in quantity from a 1% change in price

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Important Points to Note: Changes in total expenditures on a good caused by changes in price can be predicted from the price elasticity of demand –if demand is inelastic (0 > e Q,P > -1), price and total expenditures move in the same direction –if demand is elastic (e Q,P < -1), price and total expenditures move in opposite directions

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Important Points to Note: If other factors that enter the demand function (prices of other goods, income, preferences) change, the market demand curve will shift –the income elasticity (e Q, I ) measures the effect of changes in income on quantity demanded –the cross-price elasticity (e Q,P’ ) measures the effect of changes in another good’s price on quantity demanded

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Important Points to Note: There are a number of relationships among the various demand elasticities –the Slutsky equation shows the relationship between uncompensated and compensated price elasticities –homogeneity is reflected in the fact that the sum of the elasticities of demand for all of the arguments in the demand function is zero

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