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Frank Cowell: Microeconomics
Revision Lecture
EC202
http://darp.lse.ac.uk/ec202
29th April 2010
Frank Cowell
Overview...
Revision lecture
Frank Cowell: Microeconomics
Styles of
question
How to see what
you need to do
Doing short
questions
Planning
answers
Doing long
questions
Objectives of the lecture
Frank Cowell: Microeconomics
A look back at Term 1
Exam preparation
Reference materials used (1)
Exam papers (and outline answers)
2005 1(a)
2006 1(a)
2006 4
2008 1(b)
2009 1(c)
Reference materials used (2)
CfD presentations 2.9, 3.3, 8.12
All related to past exam questions
The exam paper
Frank Cowell: Microeconomics
Scope of exam material
Structure and format of paper
what’s covered in the lectures…
… is definitive for the exam
follows that of last four years
check out the rubric from, last year’s paper
Mark scheme
40 marks for question 1 (8 marks for each of the five parts)
20 marks for each of the other three questions
multipart questions: except where it’s obvious, roughly equal
marks across parts
Question style – three types
Frank Cowell: Microeconomics
1 Principles
2 Model solving
a standard framework
you just turn the wheels
3 Model building
reason on standard results and arguments
can use verbal and/or mathematical reasoning
usually get guidance in the question
longer question sometimes easier?
Examples
from past
question 1
One type not necessarily “easier” or “harder” than another
part A (question 1) usually gets you to do both types 1 and 2
type 3 is usually only in parts B and C of paper
Overview...
Revision lecture
Frank Cowell: Microeconomics
Styles of
question
How to tackle the
main types of
question
Doing short
questions
Planning
answers
Doing long
questions
2009 1(c)
Frank Cowell: Microeconomics
Straightforward
“principles” question
Just say what you
need to say
2005 1(a)
Frank Cowell: Microeconomics
Straight “principles”
Note the contrast
between firm and
consumer
Be sure to give your
reasons
2006 1(a)
Frank Cowell: Microeconomics
Principles again
But format of question
gives you a hint…
…write out
decomposition formula
Then read off results
2008 1(b)
Frank Cowell: Microeconomics
Principles and model-solving
Write down the principle
Write down the basics of the model
WARP can be stated simply in terms of “affordability”
To check whether week 2’s bundle can be afforded at week 1’s
prices (etc. etc.) we need to write down the costs
Check the on-line answers for the (short) detailed reasoning…
2006 4
Frank Cowell: Microeconomics
Straight principles can come up in long questions
Don’t ignore them in a rush to get to the model!
Compare this with CfD 8.12
CfD (from book) doesn’t have this bit, but take it seriously
There are some easy marks just writing down the definition…
…and the diagram helps you to answer part (b)
Overview...
Revision lecture
Frank Cowell: Microeconomics
Styles of
question
How to do well in
exams
Doing short
questions
Planning
answers
Doing long
questions
Planning Answers
Frank Cowell: Microeconomics
What’s the point?
See the big picture
take a moment or two..
…make notes to yourself
what is the main point of the question?
and the subpoints?
balance out the answer
imagine that you’re drawing a picture
if pressed for time, don’t rush to put in extra detail…
…you can go back
Be an economist with your own time
don’t solve things twice!
reuse results
answer the right number of questions!!!
Frank Cowell: Microeconomics
Tips
Follow the leads
Pix
help you to see the solution
help you to explain your solution to examiner
What should the answer be?
examiners may be on your side!
so if you’re pointed in the right direction, follow it…
take a moment before each part of the question
check the “shape” of the problem
use your intuition
Does it make sense?
again take a moment to check after each part
we all make silly slips
Frank Cowell: Microeconomics
Long questions
Let’s look at three examples
Illustrates two types of question
taken from exercises in the book
but of “exam type” difficulty
covered in CfD
Ex 2.9 and 3.3 are straight model solving
Ex 8.12 incorporates some model building
Look out for tips
In all three questions, use pictures to clarify solution
following hints in 3.3 [The “Explain carefully…” bits]
Overview...
Revision lecture
Frank Cowell: Microeconomics
Styles of
question
How to combine
principles and
model-solving…
Doing short
questions
Planning
answers
Doing long
questions
•CfD 2.9
•CfD 3.3
•CfD 9.12
Ex 2.9(1): Question
Frank Cowell: Microeconomics
purpose: demonstrate relationship between short and long run
method: Lagrangean approach to cost minimisation. First part can be
solved by a “trick”
Ex 2.9(1): Long-run costs
Frank Cowell: Microeconomics
Production function is homogeneous of degree 1
CRTS implies constant average cost
increase all inputs by a factor t > 0 (i.e. z → tz)…
…and output increases by the same factor (i.e. q → tq)
constant returns to scale in the long run
C(w, q) / q = A (a constant)
so C(w, q) = Aq
differentiating: Cq(w, q) = A
So LRMC = LRAC = constant
Their graphs will be an identical straight line
Ex 2.9(2): Question
Frank Cowell: Microeconomics
method:
Standard Lagrangean approach
Ex 2.9(2): short-run Lagrangean
Frank Cowell: Microeconomics
In the short run amount of good 3 is fixed
z3 = `z3
Could write the Lagrangean as
But it is more convenient to transform the problem thus
where
Ex 2.9(2): Isoquants
Frank Cowell: Microeconomics
Sketch the isoquant map
z2
z1
Isoquants do not touch the axes
So maximum problem must have an interior solution
Ex 2.9(2): short-run FOCs
Frank Cowell: Microeconomics
Differentiating Lagrangean, the FOCS are
This implies
To find conditional demand function must solve for l
use the above equations…
…and the production function
Ex 2.9(2): short-run FOCs (more)
Frank Cowell: Microeconomics
Using FOCs and the production function:
This implies
where
This will give us the short-run cost function
Ex 2.9(2): short-run costs
Frank Cowell: Microeconomics
By definition, shortrun costs are:
This becomes
Substituting for k:
From this we get
SRAC:
SRMC:
Ex 2.9(2): short-run MC and AC
Frank Cowell: Microeconomics
marginal
cost
average
cost
q
Ex 2.9(3): Question
Frank Cowell: Microeconomics
method:
Draw the standard supply-curve diagram
Manipulate the relationship p = MC
Ex 2.9(3): short-run supply curve
Frank Cowell: Microeconomics
average cost curve
marginal cost curve
minimum average cost
p
supply curve
p
q
q
Ex 2.9(3): short-run supply elasticity
Frank Cowell: Microeconomics
Use the expression for marginal cost:
Set p = MC for p ≥ p
Rearrange to get supply curve
Differentiate last line to get supply elasticity
Ex 2.9: Points to remember
Frank Cowell: Microeconomics
Exploit CRTS to give you easy results
Try transforming the Lagrangean to make it easier
to manipulate
Use MC curve to derive supply curve
Overview...
Revision lecture
Frank Cowell: Microeconomics
Styles of
question
A problem with
discontinuous
supply…
Doing short
questions
Planning
answers
Doing long
questions
•CfD 2.9
•CfD 3.3
•CfD 8.12
Ex 3.3(1) Question
Frank Cowell: Microeconomics
purpose: to derive competitive supply function
method: derive AC, MC
Ex 3.3(1) Costs
Frank Cowell: Microeconomics
Total cost is: F0 + ½ aqi2
Marginal cost: aqi
Average cost: F0/qi + ½ aqi
Therefore MC intersects AC where:
This is at output level q where:
At this point AC is at a minimum p where:
For q below q there is IRTS and vice versa
Ex 3.3(1) Supply
Frank Cowell: Microeconomics
If p > p the firm supplies an amount of output such that
If p < p the firm supplies zero output
otherwise the firm would make a loss
If p = p the firm is indifferent between supplying 0 or q
p = MC
in either case firm makes zero profits
To summarise the supply curve consists of :
Ex 3.3(1): Supply by a single firm
Frank Cowell: Microeconomics
Average cost
p
Marginal cost
Supply of output
q
qi
Ex 3.3(2) Question
Frank Cowell: Microeconomics
purpose: to demonstrate possible absence of equilibrium
method: examine discontinuity in supply relationship
Ex 3.3(2): Equilibrium?
Frank Cowell: Microeconomics
AC,MC and supply of firm
p
Demand, low value of b
Demand, med value of b
Demand, high value of b
Solution for high
value of b is where
Supply = Demand
AC
MC
qi
Ex 3.3(2) Equilibrium
Frank Cowell: Microeconomics
Outcome for supply by a single price-taking firm
High demand: unique equilibrium on upper part of supply curve
2. Low demand: equilibrium with zero output
3. In between: no equilibrium
1.
Given case 1 “Supply = Demand” implies
This implies:
But for case 1 we need p ≥ p
from the above this implies
Ex 3.3(3) Question
Frank Cowell: Microeconomics
purpose: to demonstrate effect of averaging
method: appeal to a continuity argument
Ex 3.3(3) Average supply, N firms
Frank Cowell: Microeconomics
Define average output
Set of possible values for
average output:
Therefore the average supply
function is
Ex 3.3(3) Average supply, limit case
Frank Cowell: Microeconomics
As N the set J(q) becomes dense in [0, q]
So, in the limit, if p = p average output can take
any value in [0, q]
Therefore the average supply function is
Ex 3.3(3): Average supply by N firms
Frank Cowell: Microeconomics
Average cost (for each firm)
Marginal cost (for each firm)
p
Supply of output for averaged
firms
q
q
Ex 3.3(4) Question
Frank Cowell: Microeconomics
purpose: to find equilibrium in large-numbers case
method: re-examine small-numbers case
Ex 3.3(4) Equilibrium
Frank Cowell: Microeconomics
Equilibrium depends on where demand curve is located
High demand
characterise in terms of (price, average output)
equilibrium is at (p, p/a) where p = aA / [a+b]
Medium demand
equilibrium is at (p, [A – p]/b)
equivalent to (p, bq) where b := a[A – p] / [bp]
Achieve this with a proportion b at q and 1–b at 0
Low demand
equilibrium is at (p, 0)
Ex 3.3(4): Eqm (medium demand)
Frank Cowell: Microeconomics
AC and MC (for each firm)
Supply of output (averaged)
Demand
p
Equilibrium
Equilibrium
achieved by
mixing firms at 0
and at q
b here
1b here
q*
q
q
Ex 3.4: Points to remember
Frank Cowell: Microeconomics
Model discontinuity carefully
Averaging may eliminate discontinuity problem in
a large economy
depends whether individual agents are small.
Equilibrium in averaged model may involve
identical firms doing different things
equilibrium depends on the right mixture
Overview...
Revision lecture
Frank Cowell: Microeconomics
Styles of
question
Modelling choice
under uncertainy
Doing short
questions
Planning
answers
Doing long
questions
•CfD 2.9
•CfD 3.3
•CfD 8.12
Ex 8.12(1): Question
Frank Cowell: Microeconomics
purpose: to develop an analysis of insurance where terms are less
than actuarially fair
method: model payoffs in each state-of-the-world under different
degrees of coverage. Find optimal insurance coverage. Show how
this responds to changes in wealth
Ex 8.12(1): model
Frank Cowell: Microeconomics
Use the two-state model (no-loss, loss)
Consider the person’s wealth in extremes
Suppose partial insurance is possible
if uninsured: (y0, y0 L)
if fully insured: (y0 κ, y0 κ)
if person insures a proportion t of loss L…
…pro-rata premium is tκ
So if a proportion t is insured wealth is
([1 t]y0 + t [y0 κ], [1 t][y0 L] + t [y0 κ])
which becomes (y0 tκ, y0 tκ + [1 t]L)
Ex 8.12(1): utility
Frank Cowell: Microeconomics
Put payoffs (y0 tκ, y0 tκ + [1 t]L) into the
utility function
Expected utility is
Therefore effect on utility of changing coverage is
Could there be an optimum at t =1?
Ex 8.12(1): full insurance?
Frank Cowell: Microeconomics
What happens in the neighbourhood of t = 1?
We get
Simplifying, this becomes [Lπ κ] uy(y0 κ)
positive MU of wealth implies uy(y0 κ) > 0
by assumption Lπ <κ
so [Lπ κ] uy(y0 κ) < 0
In the neighbourhood of t =1 the individual could increase
expected utility by decreasing t
Therefore will not buy full insurance
Ex 8.12(2): Question
Frank Cowell: Microeconomics
Method
Standard optimisation
Differentiate expected utility with respect to t
Ex 8.12(2): optimum
Frank Cowell: Microeconomics
For an interior maximum we have
Evaluating this we get
So the optimal t∗ is the solution to this equation
Ex 8.12(3): Question
Frank Cowell: Microeconomics
Method
Take t* as a function of the parameter y0
This function satisfies the FOC
So to get impact of y0:
Differentiate the FOC w.r.t. y0
Rearrange to get t* / y0
Ex 8.12(3): response of
*
t
to y0
Frank Cowell: Microeconomics
Differentiate the following with respect to y0:
This yields:
On rearranging we get:
Ex 8.12(3): implications for coverage
Frank Cowell: Microeconomics
Response of t* to y0 is given by
The denominator of this must be negative:
uyy(⋅) is negative
all the other terms are positive
The numerator is positive if DARA holds
Therefore ∂t*/∂y0 < 0
So, given DARA, an increase in wealth reduces the
demand for insurance
Ex 8.12: Points to remember
Frank Cowell: Microeconomics
Identify the payoffs in each state of the world
Set up the maximand
ex-post wealth under…
…alternative assumptions about insurance coverage
expected utility
Derive FOC
Check for interior solution
Get comparative static effects from FOCs