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Value-at-risk (VaR) is
a category of risk metrics that describe probabilistically the
market risk of a trading portfolio.
Value-at-risk is widely used by banks,
securities firms, commodity
merchants, energy merchants, and other trading
organizations. Such firms could track their portfolios' market risk by
using historical volatility as a
risk metric. They might do
so by calculating the historical volatility of their portfolio's
market value
over a rolling 100 trading days. The problem with doing this is that it
would provide a retrospective indication of
risk. The historical
volatility would illustrate how risky the portfolio had been over the
previous 100 days. It would say nothing about how much market risk the
portfolio was taking today.
For institutions to manage risk, they must know about risks while they
are being taken. If a trader mis-hedges a portfolio, his employer needs to
find out before a loss is incurred. Value-at-risk gives institutions the ability to
do so. Unlike retrospective risk metrics, such as historical volatility,
value-at-risk is prospective. It quantifies market risk while it is being taken.
Measure time in trading days. Let 0 be the current time.
We know a portfolio's current market value
. Its
market value
in one trading day is unknown. It is a
random variable. We may ascribe it a probability distribution. With
value-at-risk,
we summarize a portfolio's market risk by reporting some parameter of this
distribution. For example, we might report the 90%-quantile
of the portfolio's single-period USD
loss. This is called one-day 90% USD VaR. If a portfolio has a one-day 90%
USD VaR of, say, USD 5MM, it can be expected to lose more than USD 5MM on
one trading day our of ten. This is illustrated in Exhibit 1.
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One-day 90% USD VaR is illustrated for a
hypothetical portfolio. Shown is the probability density function for the
portfolio's value 1P one trading day from now. The
portfolio's current value 0p is known.
value-at-risk equals
the amount of money such that there is a 90% probability of the
portfolio losing less than that amount over the next trading day.
This is indicated in the Exhibit. |
Value-at-risk can be measured in other ways. For example, bank regulations
require that value-at-risk be calculated as a 99%-quantile of loss over a two-week
horizon. Still other metrics are possible. We could measure value-at-risk as the
standard deviation of portfolio value
or the standard deviation of portfolio return. Essentially, any parameter
of the distribution of a portfolio's future value can be used to measure
value-at-risk.
Let's formalize this. value-at-risk is applicable to any
liquid
portfolio—that is, any portfolio that can reasonably be marked to market
on a regular basis. Value-at-risk is not applicable to
illiquid assets, such as real estate or fine art. Value-at-risk considers a portfolio's performance over a specific horizon—a
trading day, two weeks, a month, etc. We call this the VaR horizon.
Value-at-risk is measured in a particular currency, USD in the
example above, but any
currency can be used. This is called the base currency.
Finally, the portfolio's market risk is summarized with a single
number. Informally, we called this a parameter of the distribution of
portfolio value. More formally, it is any function of both the portfolio's current value
and its (random) value
at the end of the VaR horizon. In
our example, the
function was the 90%-quantile of loss. As we mentioned, other functions
are possible. A VaR measurement is
the value obtained for that function for a specific portfolio at a
specific point in time.
We distinguish between a
VaR measure and a
VaR metric. A VaR measure is the procedure by
which we arrive at a VaR measurement. It is some computational algorithm,
which is typically coded on a computer.
A VaR metric is our interpretation of the VaR measurement. In our
examples, the VaR metric was one-day 90% USD VaR. Other example of VaR
metrics are:
two-week 99%
EUR VaR
one-year standard deviation of USD return
one-day
semi-variance of
JPY portfolio value
Value-at-risk became popular with trading organizations during the
1990s. It was during this period that the name "value-at-risk" entered the
financial lexicon. However, VaR measures had been used long before this.
An early user was Harry Markowitz. In his groundbreaking (1952)
paper "Portfolio Selection", he adopted a VaR metric of single period
variance of return and used this to develop techniques of portfolio
optimization. In the early 1980s, the United States
Securities and
Exchange Commission (SEC) adopted a crude VaR measure for use in assessing
the capital adequacy of
broker-dealers' trading
non-exempt
securities. A
few years later, Bankers Trust implemented a VaR measure for use with
its RAROC
capital
allocation system. During the late 1980s and early 1990s, a number of
institutions implemented VaR measures to support
capital allocation
or market risk limits.
In the early 1990s, three events popularized value-at-risk as a
practical tool for use on trading floors:
In 1993, the
Group of 30
published a groundbreaking report on derivatives practices. It was
influential and helped shape the emerging field of
financial risk
management. It promoted the use of value-at-risk by
derivatives dealers and appears
to be the first publication to use the phrase "value-at-risk."
In 1994, JP Morgan launched its free
RiskMetrics service.
This was intended to promote the use of value-at-risk among the firm's institutional
clients. The service comprised a technical document describing how to
implement a VaR measure and a covariance matrix for several hundred key
factors updated daily on the internet.
In 1995, the
Basel Committee on
Banking Supervision implemented market risk capital requirements for
banks. These were based upon a crude VaR measure, but the committee also
approved, as an alternative, the use of banks' own proprietary VaR measures in certain
circumstances.
These three initiatives came during a period of heightened
concern about systemic risks due to the emerging—and largely
unregulated—OTC derivatives market. It was also a period when a number of
organizations—including Orange County,
Barings Bank, and
Metallgesellschaft—suffered staggering losses due to speculative
trading, failed hedging programs or derivatives. Financial risk management
was a priority for firms, and value-at-risk was rapidly embraced as the tool of
choice for quantifying market risk. It was implemented by financial firms,
corporate treasuries, commodities merchants, and energy merchants.
For a detailed discussion of how value-at-risk is calculated, see
the article measuring value-at-risk.
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