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Changes in the value of
an underlier are often the primary source of
risk in a
derivatives portfolio, so there
are two Greek factor sensitivities for measuring
such risk. Delta and gamma represent first- and second-order measures of
sensitivity to an underlier.
Consider a hypothetical
portfolio whose value depends upon some underlier whose current value is
USD 101. Exhibit 1 illustrates how the current
market value of a
hypothetical portfolio depends upon the current underlier value.
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The market value in USD
MM of a
hypothetical portfolio as a function of underlier value in USD. The
current underlier value is USD 101. The graph indicates what the
portfolio value would be if the underlier had a current value
anywhere in the interval USD 99 to USD 103. |
Exhibit 1 fully describes
the relationship between the portfolio's current market value and the
underlier, assuming other market variables are unchanged. With just two
numbers—delta and gamma—we can summarize the information contained in
Exhibit 1. Certainly, two numbers cannot describe the wealth of detail
contained in a picture, but with delta and gamma we capture the two most
important pieces of information in the picture.
Let's start with delta.
The most significant information that Exhibit 1 provides about this
particular portfolio is the fact that its value will increase if the
underlier increases, and it will decrease if the underlier decreases. This
is the information that delta conveys, along with the magnitude of such
sensitivity.
If we fit a tangent line
to the curve in Exhibit 1 at the underlier's current value of 101, the
slope of that line will capture the direction and magnitude of the
portfolio's sensitivity to the underlier. Delta is the slope of that
tangent line.
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Delta is the slope of the tangent line fit
to the portfolio's value function at the current underlier value. In
this example, the current underlier value is USD 101, and the slope
of the tangent line 0.8MM. |
For example, in Exhibit
2, the slope of the tangent line is 0.8MM (for each unit increase in the
underlier, the portfolio's price appreciates by 0.8MM). Accordingly, the
portfolio's delta is 0.8MM.
Fitting tangent lines to
functions is the province of calculus, so we turn to calculus for the
formal definition of delta. Let 0 be the current time. Let
and
be current
values for the portfolio and underlier (see the
notation conventions documentation). Delta
is the first partial derivative of a portfolio's value with
respect to the value of the underlier:
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[1] |
This technical definition
leads to an approximation for the behavior of a portfolio.
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[2] |
Where
is a small change in the
underlier's current value, and
is the corresponding change in the portfolio's current value. This is called the
delta approximation.
Suppose a portfolio is
exposed to IBM stock and has a IBM delta of 1.5MM
shares. This means that,
for small market moves, the portfolio behaves like a position comprising
1.5MM shares of IBM. It will gain about USD 1.5MM if IBM stock rises USD
1 or lose about USD 3.0MM if the stock falls USD 2. Plug these numbers
into formula [2] and confirm that it is saying the same thing! Note that
the portfolio's exposure could result from direct holdings in IBM stock, a
derivatives position with IBM stock as an underlier, or some combination
of the two. If it is caused entirely by an outright position in IBM stock, then that
position must consist of exactly 1.5 million shares of IBM stock because
the delta of one unit of the underlier always equals 1.0.
If the portfolio is
exposed to several stocks, then it will have a delta for each. For
example, it's Exxon delta might be –2.5MM shares. This would behave
similarly to a short position in 2.5 million shares of American Airlines stock. If
American Airlines stock rose USD 1, the portfolio would lose about USD 2.5MM. If the
stock declined USD 0.5, the portfolio would gain about USD 1.25MM.
Now let's consider gamma.
If delta summarizes the most significant piece of information about a
portfolio's sensitivity to an underlier, gamma summarizes the second-most
significant piece of information. Delta captured the fact that the graph
in Exhibit 1 was upward sloping. It did not capture its downward
curvature. Gamma describes curvature.
Exhibit 3 shows the
best-fitting parabola for the graph of Exhibit 1:
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The parabola that best fits the
portfolio's value function. |
Note that the
best-fitting parabola does not exactly overlay the curve in Exhibit 3
because the curve is not itself a parabola. In general, the best-fitting
parabola will have the form:
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[3] |
where a, b
and c are constants determined to achieve the best fit. Gamma
equals 2c. As it turns out, for the best-fitting parabola, the
constant b is the portfolio's delta, and a can be solved for
based upon the portfolio's current market value.
Gamma not only tells us
the magnitude of curvature, but its direction as well. Positive gamma
corresponds to curvature that opens upward. Negative gamma corresponds to
curvature that opens downward.
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Positive gamma corresponds to curvature
that opens upward. Negative gamma corresponds to curvature that
opens downward. |
For a formal definition of gamma, we again turn to
calculus. Gamma is the second partial
derivative of a portfolio's value
with
respect to the value
of the underlier:
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[4] |
By incorporating gamma, we can improve our approximation
[2] for how the portfolio's value should change in response to small
changes in the underlier's value:
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[5] |
This is called the
delta-gamma
approximation.
While sensitivity to the
value of an underlier may be a significant determinant of a derivative
portfolio's risk, other sensitivities are also important. These
include sensitivities to implied volatilities, interest rates, and the
passage of time. There are Greek factor sensitivities for each of these,
called: vega,
rho,
and theta, respectively.
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A
trader has a portfolio of options on crude oil. The portfolio’s
delta is 1,300 barrels. Suppose the price of crude oil immediately
rose by USD 1.20, and all other variables remained unchanged. Use
[2] to approximate what profit/loss the trader
will realize from the move.
[solution]
The
following graphs depict, for three hypothetical portfolios, each
portfolio's market value as a function of some underlier's value.
In each graph, the underlier's current value is indicated with a
gold triangle and a vertical line. For each graph, decide if
1. the
portfolio’s delta is positive, zero or negative, and
2. the
portfolio’s gamma is positive, zero or negative.

[solution]
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delta
hedge A type of hedge that is widely used by derivative
dealers to reduce or eliminate a portfolio's exposure to some
underlier.
derivative
instrument An instrument
which derives its value from the value of other financial
instruments. Article includes a list of vanilla and exotic derivatives.
directional strategy
A trading or investment strategy that entails taking net long or short
positions in a market.
duration and convexity
Risk metrics employed in fixed income markets.
dynamic
hedging A technique that is widely used by derivatives dealers
to hedge gamma or vega exposures.
Greeks A set of
factor sensitivities, which includes delta and gamma.
option pricing theory The
body of financial theory used by financial engineers to value options and other
derivative instruments.
option spreads
Positions combining one or more options in a single underlier.
put-call
parity
A formula that relates the price of a put to the price of a
corresponding call.
rho Factor sensitivity measuring a portfolio's first order
(linear) sensitivity to the risk-free rate.
theta Factor sensitivity measuring a portfolio's first
order (linear) sensitivity to the passage of time
vega Factor sensitivity measuring a portfolio's first
order (linear) sensitivity to the implied volatility of an underlier
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Ads by Contingency Analysis
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Natenberg (1994)
and Taleb (1996)
discuss delta and gamma in the context of trading. Natenberg is
introductory. Taleb is a sophisticated book for professional
derivatives traders.
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