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This article discusses Monte Carlo and historical VaR
measures. Exhibit 1 is reproduced from the overview article
measuring value-at-risk. It indicates the
three processes that are essential to all practical
VaR measures. This article assumes
familiarity with the concepts referenced in that exhibit.
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All practical VaR measures accept
portfolio data and historical market data as inputs. They process
these with a mapping procedure, inference procedure, and
transformation procedure. Output comprises the value of a
VaR
metric. That value is the VaR measurement. |
A transformation procedure accepts two inputs:
a
portfolio
mapping function obtained from the
mapping procedure, and
a characterization of the conditional joint distribution of the
key factors obtained from the
inference procedure.
The transformation procedure must somehow combine these to characterize the
conditional distribution of the portfolio's value. Based upon that
characterization, the transformation procedure values the desired
VaR metric.
If a portfolio mapping function is a
linear or
quadratic
polynomial, then a linear
transformation or quadratic
transformation should generally be used. Both run rapidly and entail
little or no error. Monte Carlo and historical transformations take longer
to run and introduce the standard error common to all
Monte Carlo estimators. For this
reason, they are reserved for only those portfolios for which portfolio
mapping functions are neither linear nor quadratic. Such portfolios
generally contain derivatives,
mortgage-backed securities or
other complex non-linear instruments.
Let time 0 be the current time, and let time 1 be the end
of the VaR horizon (see the
notation conventions documentation). A
portfolio mapping
is obtained from a mapping procedure, where
 is the random variable for the
portfolio's value at the end of the VaR horizon,
 is the
portfolio mapping function, and
 is the
key vector.
Based upon the characterization of the joint distribution
of obtained from the inference procedure, a
Monte Carlo transformation procedure generates several thousand
pseudorandom realizations
for . Based upon these, it calculates
corresponding realizations
.
The histogram of these realizations
provides a discrete approximation for the distribution of
.
Based on this, any reasonable VaR metric can be valued. Exhibit 2
illustrates a histogram of portfolio value realizations from an actual
Monte Carlo VaR analysis.
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Histogram of portfolio values obtained
from an actual VaR analysis. A sample size of 5000 realizations was
used. The skewness of the distribution suggests positive
gamma. |
Because it depends upon the
Monte Carlo method, a Monte
Carlo transformation procedure entails standard error. The magnitude of
the standard error depends upon many things, including:
the VaR metric,
the portfolio, and
the sample size used in the Monte Carlo analysis.
A crude rule of thumb is that the standard error will be
about 1% of calculated VaR for a typical portfolio if a
quantile-of-loss VaR metric is used
and the sample size is 10,000. Because valuing a portfolio mapping
function 10,000 times can be an enormous computational load, most
organizations don't use nearly as large a sample size. Sample sizes as low
as 500 are commonly used. Because the standard error of a Monte Carlo
analysis is proportional to the square root of the sample size, such low
sample sizes introduce standard errors on the order of 4.5%. (I know of
several software vendors who obfuscate this fact in their product
literature.)
A solution to this problem is to employ variance
reduction. Techniques based upon
control variates and
stratified sampling were
published by Cárdenas, et al. (1999). Both methods
they propose employ a quadratic
remapping
for . The remapping
does not replace
. Instead, it is used to facilitate
variance reduction so VaR can be more easily calculated for
.
With the method of control variates,
is used as a control variate for
. For this purpose,
we need to calculate the VaR of
,
but this is easily accomplished with a
quadratic
transformation procedure. Variance reduction is excellent for most
portfolios and VaR metrics.
With the method of stratified sampling,
is used to construct a stratification. The methodology varies depending
upon the VaR metric. For a quantile of loss VaR metric, realizations
of 1R are stratified into two regions:
one comprising realizations
such that
exceeds the VaR of
,
and
the other comprises realizations
such that
is less than or equal to the VaR of
.
Variance reduction is excellent for most portfolios and
VaR metrics.
To further improve variance reduction, the above two
methodologies can be combined. Cárdenas, et al. (1999)
also suggested a technique of selective valuation that can be used. While
this is technically not a variance reduction technique, it has largely the
same effect. See Holton (2003)
for a detailed discussion of all these methodologies.
Historical
transformations are identical to Monte Carlo transformations
except for one difference. Both employ the Monte Carlo method to construct
a histogram of realizations
of . The
difference lies in how they construct realizations
for .
Monte Carlo transformations randomly generate them based upon a
characterization of the distribution of
.
Historical transformations employ realizations
constructed from historical market data for
.
Historical transformations were popular during the early
1990s because they were intuitively easy to explain to non-technical
professionals—VaR was being calculated based on one-day profit or losses
that the portfolio would have realized based on market movements that
occurred during each of the past 500 or so trading days. There are,
however compelling reasons to avoid historical transformations. First,
because they are based on the Monte Carlo method, they entail exactly the
same standard error as Monte Carlo transformations. While Monte Carlo
transformations can minimize standard error by using large sample sizes,
this is not possible for historical transformations. Their sample sizes
are limited by the availability of relevant historical market data. It is
rare that historical transformations have sample sizes grater than 1000,
so standard errors is generally significant. Historical transformations
are not amenable to many of the powerful methods of variance reduction
available for Monte Carlo transformations. Finally, use of historical
realizations introduces biases relating to
conditional heteroskedasticity.
See Holton (2003).
VaR measures are called
Monte Carlo VaR measures or
historical VaR measures if they
employ, respectively, Monte Carlo or historical transformations. With
Monte Carlo VaR measures, an inference procedure typically characterizes
the distribution of
by
assuming some standard joint distribution—such as the
joint-normal distribution—and
specifying a covariance matrix and
mean vector for this. Based on this
characterization, the Monte Carlo transformation randomly generates
realizations
.
Historical VaR measures are somewhat unique in how their inference
procedures characterize the distribution of
.
Literally, they do so with the set of historical realizations
.
The inference procedure directly passes this set of historical
realizations to the historical transformation. The set of historical
realizations
is the inference procedure's characterization of the joint distribution of
.
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Ads by Contingency Analysis
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Marrison (2002)
is an elementary text that introduces Monte Carlo and historical
VaR measures in the context of bank risk management. Holton (2003) is the definitive text on value-at-risk.
It covers Monte Carlo transformations and the use of variance
reduction in in such transformations. Glasserman (2003)
also discusses variance reduction in Monte Carlo transformations,
presenting a different techniques.
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Cárdenas, Juan,
Emmanuel Fruchard, Jean-François Picron, Cecilia Reyes, Kristen
Walters, and Weiming Yang (1999). Monte Carlo within a day,
Risk, 12 (2), 55-59. |
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