Demand Relationships among Goods

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1 Demand Relationships among Goods
© 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

2 The Plan Cross-price effects Substitutes and complements
Composite commodities © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

3 The Two-Good Case How the quantity of x chosen might be affected by a decrease in the price of y Shifting the budget constraint outward The quantity of good y chosen has increased In (a), small substitution effect x and py move in opposite directions In (b), large substitution effect x and py move in the same direction © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

4 Differing Directions of Cross-Price Effects
6.1 Differing Directions of Cross-Price Effects Quantity of x (a) Quantity of y Quantity of x (b) Quantity of y I1 I1 U1 U1 U0 U0 I0 I0 y1 y1 x1 x1 y0 x0 y0 x0 In both panels, the price of y has decreased. In (a), substitution effects are small; therefore, the quantity of x consumed increases along with y. Because ∂x/∂py < 0, x and y are gross complements. In (b), substitution effects are large; therefore, the quantity of x chosen decreases. Because ∂x/∂py > 0, x and y would be termed gross substitutes. © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

5 The Two-Good Case In elasticity terms Slutsky-type equation
Substitution effect (+) Income effect (-) if x is normal Combined effect (ambiguous) Just expand sufficiently with py/x on both sides, etc. etc. & uncompensated elasticity and compensated elasticity are similar if income effect is small (=last RHS term) In elasticity terms © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

6 6.1 Another Slutsky Decomposition for Cross-Price Effects
The cross-price effect of a change in y prices on x purchases: Cobb-Douglas utility Uncompensated demand function: x(px,py,I)=0.5 I/px Compensated demand function: xc(px,py,V)=Vpy0.5px-0.5 Marshallian demand function in this case yields: ∂x/∂py=0 ! © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

7 6.1 Another Slutsky Decomposition for Cross-Price Effects
The cross-price effect of a change in y prices on x purchases Changes in the price of y do not affect x purchases Because the substitution and income effects of a price change are precisely counterbalancing Substitution effect: © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

8 6.1 Another Slutsky Decomposition for Cross-Price Effects
Income effect: Total effect of a change in the price of y: © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

9 Substitutes and Complements
Many goods Generalize the Slutsky equation for any two goods xi, xj An elasticity relation © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

10 Substitutes and Complements
If one good may, as a result of changed conditions, replace the other in use Substitute for one another in the utility function Complements Goods that ‘‘go together’’ Complement each other in the utility function Query: Provide a few examples for both. © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

11 Substitutes and Complements
Gross substitutes Two goods, xi and xj, are said to be gross substitutes if: ∂xi/∂pj>0 An increase in the price of one good causes more of the other good to be bought © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

12 Substitutes and Complements
Gross complements Two goods, xi and xj, are said to be gross complements if: ∂xi/∂pj<0 An increase in the price of one good causes less of the other good to be purchased © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

13 Substitutes and Complements
Asymmetry of the gross definitions Gross definitions of substitutes and complements - are not symmetric It is possible, by the definitions For x1 to be a substitute for x2 And at the same time For x2 to be a complement of x1 © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

14 6.2 Asymmetry in Cross-Price Effects
Utility function, U(x,y) = ln x + y The Lagrangian, ℒ = ln x + y + λ(I – pxx – pyy) First-order conditions ∂ℒ /∂x = 1/x - λpx = 0 ∂ℒ /∂y = 1 - λpy = 0 ∂ℒ /∂λ= I - pxx - pyy = 0 So, pxx = py © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

15 6.2 Asymmetry in Cross-Price Effects
Substitution into the budget constraint Solve for the Marshallian demand function for y I=pxx+pyy = py+pyy So, y=(I-py)/py An increase in py - must decrease spending on y Because px and I are unchanged, spending on x must increase ∂x/∂py>0, gross substitutes ∂y/∂px=0, independent (no complement!) © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

16 Net (Hicksian) Substitutes and Complements
Goods xi and xj are said to be net substitutes if: Goods xi and xj are said to be net complements if: © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

17 Net (Hicksian) Substitutes and Complements
These definitions Look only at the substitution terms to determine whether two goods are substitutes or complements Look only at the shape of the indifference curve This definition is unambiguous because the definitions are perfectly symmetric (why?) Youngs theorem: E_ij = E_ji © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

18 Composite Commodities
In the most general case An individual who consumes n goods Will have demand functions that reflect n(n+1)/2 different substitution effects Query. Verify this formula for n=2 Convenience: group goods into larger aggregates Examples: food, clothing, “all other goods” S11, s12, s21, s22 – but s12 = s21 due to Young’s theorem -> 3 different substitution effects = 2(2+1)/2=3. © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

19 Composite Commodities
Consumers choose among n goods The demand for x1 will depend on the prices of the other n-1 commodities (why?) If all of these prices move together: lump them into a single composite commodity, y Let p20…pn0 represent the initial prices of these other commodities Assume they all vary together (so that the relative prices of x2…xn do not change) Due to budget constraint © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

20 Composite Commodities
Define the composite commodity y To be total expenditures on x2…xn at the initial prices, y = p20x2 + p30x3 +…+ pn0xn The individual’s budget constraint is I = p1x1 + p20x2 +…+ pn0xn = p1x1 + y Assumption; all of the prices p20…pn0 change by same factor t (t > 0) © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

21 Composite Commodities
Budget constraint (after changes in prices: 0 to 1) I = p1x1 + tp20x2 +…+ tpn0xn = p1x1 + ty Changes in p1 or t induce substitution effects As long as p20…pn0 move together We can confine our examination of demand to choices between buying x1 and “everything else” © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

22 Composite Commodities
A composite commodity A group of goods for which all prices move together These goods can be treated as a single commodity The individual behaves as if he is choosing between x1 and the entire composite group © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

23 6.3 Housing Costs as a Composite Commodity
Suppose that an individual receives utility from three goods: Food (x) Housing services (y), measured in hundreds of square feet Household operations (z), measured by electricity use CES utility function Query. Determine the elasticity of substitution. Delta = -1, sigma = 1/(1-delta) = 1/2 © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

24 6.3 Housing Costs as a Composite Commodity
Lagrangian technique Can be used to calculate Marshallian demand functions for these goods as © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

25 6.3 Housing Costs as a Composite Commodity
If initially I = 100, px = 1, py = 4, pz = 1 The demand functions predict: x* = 25, y* = 12.5, z* = 25 $25 is spent on food and $75 is spent on housing-related needs Assume that the py and pz move together Define the “composite commodity” housing (h) h = 4y + 1z (expenses, where ph=1, initially) Initial quantity of housing = total spent on housing (75) Observe that ph = 1 (defined!) arbitrarily, each different normalization would do as well! © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

26 6.3 Housing Costs as a Composite Commodity
py and pz always move together ph will always be related to these prices, ph=pz=0.25py Recalculate the demand function for x as a function of I, px, and ph: As t ph = t pz (move together), ph = pz As t py = 4 ph -> ph = ¼ py If I = 100, px = 1, py = 4, and ph=1, x*=25, h* =75 Query. What happens if ph=2 (rises)? © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

27 Theory of two-stage budgeting
Partition of goods Into m nonoverlapping groups (r =1, m) A separate budget (lr) Devoted to each category Demand functions for the goods within any one category Depend on the prices of goods within the category And on the category’s budget allocation © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.

28 Theory of two-stage budgeting
Demand is given by: xi(p1,…pn,I)=xiЄr(piЄr,Ir), r=1,m © 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.