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Asset Allocation: Understanding Portfolio Optimization and Risk Management, Slides of Banking and Finance

An outline of a talk on asset allocation, discussing the importance of asset allocation decisions, developments in the theory, and new approaches using ideas from derivative pricing. It also covers numerical implementation using monte carlo and ways to speed up the process. The scope of this approach to asset allocation is also addressed.

Typology: Slides

2012/2013

Uploaded on 07/29/2013

sathyanna
sathyanna 🇮🇳

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Outline of Talk
Introduction and background
Importance of Asset Allocation Decision
Developments in the Theory Markowitz , Merton
New approaches uses ideas from derivative pricing
Very brief outline
Numerical implementation using Monte Carlo
Ways to speed up Monte Carlo
Scope of this approach to asset allocation
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Outline of Talk

  • Introduction and background
  • Importance of Asset Allocation Decision
  • Developments in the Theory Markowitz , Merton
  • New approaches uses ideas from derivative pricing
  • Very brief outline
  • Numerical implementation using Monte Carlo
  • Ways to speed up Monte Carlo
  • Scope of this approach to asset allocation

2

BACKGROUND

  • Asset allocation is key investment decision
  • Becoming more important for individuals: Trend to defined contribution pension plans in many counties
  • Fallout from the Enron collapse
  • Stock bond relative weights more important than which stocks
  • About 90% of performance of pension funds is determined by asset allocation decision
  • Rule of thumb 60% in stocks 40% in bonds
  • In practice managers change the weights over time
  • Can theory help? Is there any scientific guidance?

3

What does your broker tell you?

Asset allocation recommendations (Merrill Lynch)

Investor type Cash Stock Bonds Conservative 20% 45% 35% Moderate 5% 55% 40% Aggressive 5% 75% 20%

Where do these numbers come from?

5

Features of Asset Allocation Model

  • Asset Classes
  • Dynamics of these assets
  • Relevant state variables
  • Estimate parameters
  • Model of investor’s preferences
  • Theoretical formulation as optimization problem
  • Numerical implementation

6

Merton’s Approach

  • Under some assumptions solved for the portfolio weights
  • Investor hedges against changes in the opportunity set
  • Interest rate changes: changes in the market price of risk
  • If interest rate constant and MPR constant then just mean variance component
  • Important special case: lognormal stocks, special utility function: exact solution
  • Hard to implement Merton’s dynamic programming approach for general assumptions, many assets and sensible constraints

8

Martingale Approach

  • Powerful approach Cox Huang(1989), Karatzas, Lehoczky and Shreve(1997) Pliska
  • Use ideas from option pricing to simplify the optimization problem
  • Dynamic problem is reduced to static problem
  • This approach gives the dynamics of the investor’s optimal wealth
  • But we need actual portfolio weights to instruct our brokers what to do

9

Markowitz

  • Markowitz showed how to select optimal portfolios
  • Assume n risky assets: One period model
  • Investors only care about expected return and variance
  • He solved for the optimal portfolio weights
  • Quadratic programming problem
  • Birth of modern portfolio theory
  • Earlier work by Bruno De Finetti

11

Optimization Problem

M in{ 1 2

πT^ V π | μT^ π = Ep, lT^ π = 1} (1)

where

  • μ is vector of expected returns
  • V is covariance matrix
  • Ep is expected portfolio return
  • π vector of portfolio weights

12

Maximizing Expected Utility

  • Assume investor has increasing concave utility function u(·)
  • Let π be fraction of initial wealth(X 0 ) in the risky asset
  • Let R(ω) denote(random) rate of return on risky asset and rf denote riskless rate
  • Investor maximizes(over π) using his or her subjective probabilities

E[ u(X 1 (ω)) ] = E[ u( X 0 [ (1 + rf ) + π(R(ω) − rf ) ] ) ] (2)

  • Expectation is taken over P -measure : P (ω)
  • First order condition

EP^ [ u

′ (X 1 ) (R − rf )] = 0

14

Cox Huang Approach

  • Cox and Huang showed how to solve the portfolio optimization problem using ideas from martingale pricing
  • Traditional approach: maximize expected utility of terminal wealth over the P measure and thus obtain optimal portfolio weights.
  • Recall optimization problem under classic approach
  • Maximize over π

EP^ [ u(X 1 (ω)) ] = EP^ [ u( X 0 [ (1 + rf ) + π(R(ω) − rf ) ] ) ]

  • This approach in continuous time can get tough so · · ·

15

Step One: Attainable Wealth

Recall that we assume there is no arbitrage. Let us denote the attainable terminal wealth by X given initial wealth X 0. We know from the no arbitrage result that

EQ^ [ (^) 1 +X r f

] = X 0

where Q is the risk neutral measure. The characterization of the set of attainable wealths is

X = {X : EQ^ [ (^) 1 +X r f

] = X 0 }

17

Step Two: Optimal Attainable Wealth

We seek to maximize the expected utility of final wealth over the set X.

max X { EP^ [u(X)] subject to EQ^ [ X 1 + rf

] = X 0 }

We solve this problem by introducing a Lagrange multipler λ.

max X { EP^ [u(X)] − λ( EQ^ [ (^) 1 +X r f

] − X 0 )}

Last expression involves expectations over P and over Q.

18

Optimal Attainable Wealth

For discrete state space this can be rewritten as

max X {

∑^ J

1

P (ωj )[u(X(ωj )) − λ(L(ωj ) X1 +(ω rj^ ) f

− X 0 ) ]}

First order conditions

P (ωj )u

′ (X(ωj )) = P (ωj )λ 1 +L(ω rj^ ) f

f or j = 1 · · · J.

become

u

′ (X(ω)) = λ 1 +L(ω r) f

f or all ω ∈ Ω.

Since u′ is a strictly decreasing continuous function it has an inverse h: so the last equation becomes ............

20

Optimal Attainable Wealth

X = h(λ (^) 1 +L r f

We still have the Lagrange multiplier. To get rid of it we use the relationship

EQ^ [

h(λ (^) 1+Lrf ) 1 + rf^ ]^ =^ X^0 (3)

Call the solution λˆ. Hence we have

Xˆ = h(λˆ L 1 + rf^ )

21