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The distinction between risk measures and risk metrics was first
made by Glyn Holton in his book
Value-at-Risk: Theory
and Practice. Briefly,
duration, delta,
beta, and volatility are all examples of
risk metrics. Any procedures by which we calculate these are risk
measures. The distinction is important because there is not a one-to-one
correspondence between risk measures and risk metrics. There are, for
example, different ways that the volatility of a portfolio might be
calculated. Each represents a different risk measure for the single risk
metric volatility.
Generally, a measure is an operation for
assigning a number to something. A metric is
our interpretation of the assigned number.
Consider an example.
Have someone take off his shoes and stand with his back
against a wall that is marked with a scale of inches. Note the
number of inches correspond to the top of the person's head. This process
is a measure. We interpret the
number obtained as the person's height. "Height" is a metric.
Here is another example. A hollow glass cylinder is a
meter long and has an internal diameter of a centimeter. It is open at one
end, sealed at the other and graded with millimeter measurements. A bath
of mercury is maintained at a temperature of
.
The cylinder is immersed in the bath, allowing it to fill with mercury.
While keeping the open end immersed, the cylinder is held vertically, with
the sealed end about 950 centimeters above the surface of the mercury. A
column of mercury is suspended within the cylinder. Its height is measured
in centimeters and is multiplied by 0.01316. The result is recorded. This
procedure is a measure. We interpret the recorded number as air
pressure measured in atmospheres. The interpretation is a metric.
When we apply a measure, the number obtained is a measurement.
Measures are employed to quantify many things: height,
temperature, aptitude, speed, consumer confidence, etc. All of these
notions being quantified are
metrics. The operations with which we quantify them are measures. There
are many metrics of risk—volatility, delta, gamma, duration, convexity,
beta, etc. We call these risk metrics. A
measure that supports a risk metric is called a
risk measure. The value obtained from applying a risk measure is a
risk measurement.
Risk measures tend to be categorized according to the
risk metrics they support. There are measures of duration, measures of delta,
etc. This is an important point. We do not categorize risk measures
according to the specific operations they entail. Operationally, there are
many different ways we might arrive at a measurement of a portfolio's
volatility. Irrespective of the actual operations, all of them are
measures of volatility. All support volatility as a risk metric.
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