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The
structural credit risk model
is a model for assessing credit risk,
typically of
a corporation's debt. It is also sometimes called the
Merton model or asset value model . It was proposed in Black and Scholes' (1973)
seminal paper on option pricing
and a more detailed paper by Merton (1974).
Merton anticipated the model in Merton (1970),
and he actively supported the work of Black and Scholes, which is why the model is
often called the Merton model. A
popular implementation of the model is the commercial
KMV model. KMV was a boutique software firm
that is now owned by Moodys.
The model considers a corporation financed through a
single debt and a single equity issue. The debt comprises a
zero-coupon
bond that matures at time t = t*, at which time it is to pay
investors b dollars. The equity pays no
dividends.
An unobservable process V describes the
firm's value
0 at any time t. We ascribe the outstanding debt and equity values
and
,
respectively. Accordingly, at any time t
At time t*, the firm's debt matures. At that
time, either
will exceed the bond's maturity value b, or it won't. In the former
case, the firm will pay off the bondholders. The remaining value of the
firm will belong to the equity holders, so
=
– b |
[2] |
In the latter case, the firm defaults on its debt. The
bondholders take ownership of the firm, and the
stockholders are left
with nothing:
= 0 |
[3] |
Combining the above two results, we obtain a general
expression for the value of the firm's
stock at the maturity of its debt:
= max(
– b, 0) |
[4] |
Look closely at this formula. It is precisely the
payoff of a
call
option on the firm's value
with strike price b. Based upon this
realization, the asset value model treats the firm's equity as a call
option on the value of the firm struck at the maturity value b of
its debt. By put-call parity, the firm's
debt comprises a risk-free bond that guarantees payment of b plus a
short put option on the value of the firm struck
at b. Accordingly
= b – max( b –
,
0) |
[5] |
The asset value model treats
just like any underlier. It assumes
follows a geometric Brownian motion with
volatility
. Further, it
makes all the other simplifying assumptions of the Black-Scholes (1973)
option pricing formula. Accordingly, the firm's equity can be valued at
any time t as
= c(
,
b,
, r,
t* – t ) |
[6] |
where c is the
Black-Scholes formula for the value of a call option, and r is
the risk-free rate. By [5], we can similarly value the
firm's debt as
= b
– p(
,
b,
, r,
t* – t ) |
[7] |
where p is the Black-Scholes formula for the value
of a put. Note that we discount the payment b at the risk free rate
because that payment is risk-free in formula [5]—we
have stripped out the credit risk as a put option.
At any time t, the
distance to default for a the firm's
debt is defined as
(
–
b) /
 |
[8] |
This is a metric indicating how many standard deviations the
equity holders' call option is
in-the-money. The smaller
the distance to default, the more likely a default is to occur. The
probability of default is precisely the probability of the call option
expiring out-of-the-money.
This is approximately equal to one minus the option's normalized
delta (if investors were
risk neutral, equality would be exact).
See this glossary's article Black-Scholes (1973)
option pricing formula for the Black-Scholes formula for delta. To
normalize that value, divide the delta by the underlier's value.
Three shortcomings of the asset value model are:
1. Its assumption that the firm's debt financing consists
of a one-year zero-coupon bond is, for most firms, an
oversimplification..
2. The Black-Scholes (1973)
simplifying assumptions are questionable in the context of corporate debt,
and
3. The firm's value
is not observable, which makes assigning values to it and its volatility
problematic.
Still, the model provides a useful context for considering
and modeling credit risk. Practical implementations of the model are used
by financial institutions and institutional investors. These extend the
model in some manner to facilitate the assigning of values to
and . Such
techniques generally relate
to the observable market capitalization of the firm.
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credit derivative
A derivative instrument designed to transfer credit risk from one
party to another.
credit risk Risk due to
uncertainty in a counterparty's ability to meet its obligations.
default model A type of model that assess the likelihood of default by
an obligor.
intensity model
A type of default model.
portfolio credit risk
Credit risk associated with a portfolio of obligations, typically
of multiple obligors.
pre-settlement risk Credit risk of default on a derivative instrument
prior to final settlement. |
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Crouhy et al. (2001)
and Saunders and Allen (2002)
both offer accessible introductions to asset value models. Bluhm
et al. (2002)
is the essential text. Duffie and Singleton (2003)
provide an authoritative introduction to and comparison of asset
value models and intensity (reduced form) models. Schonbucher (2003)
is the definitive book on credit derivative pricing. He discusses
various modeling techniques, including asset value models.
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Ads by Contingency Analysis
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Black,
Fischer and Myron S. Scholes (1973). The pricing of options and
corporate liabilities, Journal of Political Economy, 81, 637-654.
Merton, Robert C. (1970). A dynamic general equilibrium model
of the asset market and its application to the pricing of the
capital structure of the firm, unpublished manuscript. Available in Merton (1990).
Merton, Robert C. (1974).
On the pricing of corporate debt: the risk structure of interest
rates, Journal of Finance, 29, 449-470. Available in Merton (1990).
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