Partial Equilibrium

Christopher Makler

Stanford University Department of Economics

Econ 50: Lecture 14

Fundamental Economic Questions

What, as a society, do we produce?

Who gets what?

How do we decide?

If you were an omniscient
"social planner" in charge of everything, how would you
make these decisions?

Unit I: Autarky

How do billions of people
coordinate their economic activities?

What does it mean to
"let the market decide"
what to produce?

Responding to Prices

Unit 2: Consumer Theory

Firms face prices and
choose how much to produce

Consumers face prices and
choose how much to buy

Unit 3: Theory of the Firm

Today's Agenda

Set market demand 
equal to market supply 
to solve for the equilibrium price

Derive market supply from individual firms' supply functions

Derive market demand from individual consumers' demand

Equilibria in general, and competitive equilibria in particular

Competitive Equilibrium

Consumers and producers are small relative to the market
(like an individual firefly)
and make one decision: how much to buy or sell at the market price.

Equilibrium occurs when
the market price is such that
the total quantity demanded
equals the total quantity supplied 

Equilibrium in General

Definition 1: a situation which economic forces are "balanced"

Definition 2: a situation which is
self-replicating: \(x = f(x)\)

Transition dynamics: excess demand and supply

Stability of Equilibria

All forces can be in balance in different ways.

Assumptions of
Perfect Competition

Perfect information

Homogeneous good

Lots of buyers and sellers

Free entry and exit

Individual and Market Demand

Individual demand curve, \(d^i(p)\): quantity demanded by consumer \(i\) at each possible price

Market demand sums across all consumers:

\displaystyle D(p) = N_Cd(p)
\displaystyle D(p) = \sum_{i=1}^{N_C}{d^i(p)}

If all of those consumers are identical and demand the same amount \(d(p)\):

There are \(N_C\) consumers, indexed with superscript \(i \in \{1, 2, 3, ..., N_C\}\).

Market demand curve, \(D(p)\): quantity demanded by all consumers at each possible price

Market demand sums across all consumers:

\displaystyle D(p) = \sum_{i=1}^{N_C}{d^i(p)}

Individual and Market Supply

Firm supply curve, \(s^j(p)\): quantity supplied by firm \(j\) at each possible price

Market supply sums across all firms:

\displaystyle S(p) = N_Fs(p)
\displaystyle S(p) = \sum_{j=1}^{N_F}{s^j(p)}

If all of those firms are identical and supply the same amount \(s(p)\):

There are \(N_F\) competitive firms, indexed with superscript \(j \in \{1, 2, 3, ..., N_F\}\).

Market supply curve, \(S(p)\): quantity supplied by all firms at each possible price

Calculating Partial Equilibrium

\displaystyle \sum_{j=1}^{N_F}{s^j(p)}
\displaystyle \sum_{i=1}^{N_C}{d^i(p)}

Price \(p^*\) is an equilibrium price in a market if:

1. Consumer Optimization: each consumer \(i\) is consuming a quantity \(x^i(p^*)\) that solves their utility maximization problem.

2. Firm Optimization: each firm \(j\) is producing a quantity \(q^j(p^*)\) that solves its profit maximization problem.

3. Market Clearing: the total quantity demanded by all consumers equals the total quantity supplied by all firms.

\displaystyle \sum_{j=1}^{N_F}{s^j(p)}
\displaystyle \sum_{i=1}^{N_C}{d^i(p)}

Important Note: Three Kinds of “=" Signs

1. Mathematical Identity: holds by definition

2. Optimization condition: holds when an agent is optimizing 

3. Equilibrium condition: holds when a system is in equilibrium

MRS = {MU_1 \over MU_2}
MRS = {p_1 \over p_2}

Endogenizing the Price

Previously: agents took the price
"as given" (exogenous) - it was determined outside the model

Today: we endogenized 
the market price by analyzing the model where it's determined

Is this the “right" price?

If you were an omniscient social planner, could you do "better"
than the price the market "chooses"?

The Social Planner's Problem

Suppose you were in charge of the economy.

How would you answer the fundamental economic questions about a particular good?

How to produce it?

Want to produce any given quantity Q
at the lowest possible cost

Who gets to consume it?

How much to produce?

Want to distribute any given quantity Q
to the people who value it the most

Want to choose the quantity Q*
to maximize total surplus 
(benefit to consumers minus costs of production)

Model 1: One Consumer, One Firm



Quasilinear utility function:

u(x_1,x_2) = 10x_1 - {1 \over 2}x_1^2 + x_2

Good 2 is "dollars spent on other goods"

p_2 = 1\text{, 1 util = 1 dollar}

Total benefit (in dollars)
from \(x_1\) units of good 1:

v(x_1) = 10x_1 - {1 \over 2}x_1^2

Total cost function:

c(q) = q + {1 \over 4}q^2

Note: variable costs only


(total benefit, in dollars)

v(x_1) = 10x_1 - {1 \over 2}x_1^2

Marginal benefit,
in dollars per unit:

v'(x_1) = 10 - x_1

(also MRS, marginal willingness to pay)



c(q) = q + {1 \over 4}q^2

Marginal cost,
in dollars per unit:

c'(q) = 1 + {1 \over 2}q

What is the optimal quantity \(Q^*\) to produce and consume?



Total benefit:

v(Q) = 10Q - {1 \over 2}Q^2

Total cost:

c(Q) = Q + {1 \over 4}Q^2

Total welfare: 

Marginal welfare from producing another unit:



Marginal welfare,
in dollars per unit:

Total benefit to consumers minus total cost to firms

Marginal benefit to consumers minus marginal cost to firms

How do competitive markets
solve this problem?



Maximize net consumer surplus

v(x_1) - e(x_1) = 10x_1 - {1 \over 2}x_1^2 - p_1x_1

Maximize profits

r(q) - c(q) = pq - \left[q + {1 \over 4}q^2\right]



Net benefit from buying \(Q\) units at price \(P\):

v(Q) - P \times Q

Net benefit from selling \(Q\) units at price \(P\):

P \times Q - c(Q)

Total welfare: 

Marginal welfare from producing another unit:

Model 2: Two Consumers, Two Firms



A = number of sandwiches for Adam

v^A(A) = 8 \ln A
v^E(E) = 4 \ln E

S = number of sandwiches produced by Subway

E = number of sandwiches for Eve

T = number of sandwiches produced by Togo's

c^S(S) = S^2
c^T(T) = 2T^2

How can we choose A, E, S, and T to maximize total benefit minus total cost

subject to the constraint that the total amount produced is the total amount consumed?

"Individual ambition serves the common good." - Adam Smith

If there is a single price in the market that all consumers pay, and all producers receive, and all consumers and producers are “price takers,” then:

Every consumer sets MB = p:

  • Everyone’s MB from the last unit bought is the same.
  • Cannot increase total benefit by reallocating the good from one consumer to another

Every firm set MC = p:

  • Every firm’s MC from the last unit produced is the same.

  • Cannot reduce total costs by reallocating production from one firm to another

The MB of the last unit consumed by some person
equals the MC of the last unit produced by some firm

What can go wrong?