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Duration and convexity are
factor sensitivities that describe exposure to parallel
shifts in the spot curve. They can be applied to individual
fixed income instruments or to entire fixed income portfolios.
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Duration assesses exposure to
parallel shifts in the spot curve. It cannot warn of exposure to more
complex movements in the spot curve, including tilts and bends. |
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The idea behind duration is simple. Suppose a
portfolio has a duration of 3 years. Then that portfolio's value
will decline about 3% for each 1% increase in interest ratesor
rise about 3% for each 1% decrease in interest rates. Such a portfolio
is less risky than one which has a 10-year duration. That
portfolio is going to decline in value about 10% for each 1% rise in
interest rates. Convexity provides additional risk information.
Exhibit 2 illustrates how the
price of a fixed income portfolio might respond to parallel shifts in the spot
curve.
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The fractional change in a fixed income
portfolio's value is graphed as a function of parallel shift in the
spot curve. |
Here,
represents an immediate parallel shift in interest rates (see the
notation conventions documentation). For example,
= .015 corresponds to a 1.5% (or 150 basis point) parallel rise in the
spot curve. The variable
is the dollar change in the portfolio's value corresponding to the shift
in interest rates. Accordingly,
is the fractional change in the portfolio's value. Note that we use
preceding superscripts 0 to indicate quantities are as of the current time
0—shifts in the spot curve are considered instantaneous.
Exhibit 2 fully describes the portfolio's sensitivity to parallel
shifts in the spot curve. There is no more information that we could add
to the picture. What we try to do with duration and convexity is summarize
the entire picture of Exhibit 2 with just two numbers. Certainly, two
numbers can not describe the wealth of detail contained in a picture, so
what we do is take the two most important pieces of information in the
picture. Those two pieces of information are duration and convexity.
Let's start with duration. The most significant information
Exhibit 2
provides us about this particular portfolio is the fact that its value
will decline if interest rates rise—and rise if interest rates fall. This
is the information that duration conveys, along with the magnitude of such
sensitivity.
If we fit a tangent line to the curve in Exhibit 2, it will capture the
direction and magnitude of the portfolio's sensitivity to interest rates.
For small changes in interest rates, the line and the curve almost
overlap.
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A tangent line is fit to the curve of
Exhibit 2. Duration is the slope of the curve multiplied by minus
one. |
Duration is defined to be the slope of that tangent
line, multiplied by negative one. For example, in Exhibit 3, the
slope of the tangent line is 2.5 (for each .01 shift in
,
shifts about –.025). The portfolio's duration is 2.5 years.
Tangent lines are the province of calculus, so we turn to
calculus for the formal definition. Duration is a
weighted partial derivative:
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[1] |
This leads to the approximation
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[2] |
For example, suppose a portfolio has a duration of 5 years. That
portfolio will appreciate about 5% for each 1% decline in rates. It will
depreciated about 5% for each 1% rise in rates. It is as simple as that.
Suppose a portfolio has a duration of –2 years. The portfolio's value
will rise about 2% for every 1% rise in rates. It will decline about 2%
for each 1% decline in rates.
Approximation [2] is the primary reason people use duration. With a single
number, it summarizes a bond or a portfolio's sensitivity to changes in
interest rates.
Typically, a bond's duration will be positive.
However, instruments such as IO
mortgage backed securities have negative durations. You can also achieve a
negative duration by shorting fixed income instruments or paying fixed for
floating on an interest rate swap.
Inverse
floaters tend to have large positive durations. Their values change
significantly for small changes in rates. Highly leveraged fixed-income
portfolios tend to have very large (positive or negative) durations.
For portfolios whose cash flows are all fixed (for example, a portfolio
of
non-callable bonds) there is a
particularly simple way to calculate duration. For such portfolios,
duration is just the average maturity of the of the cash flows.
Specifically, assume a portfolio has fixed cash flows
,
each occurring at some time
years from time 0. Let 0pv( )
denote the present value at time 0 of the cash flow
,
then the duration is
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[3] |
Now the name
"duration" should make more sense, as should the fact that duration is
measured in years! When duration is calculated in this way, it is called
Macaulay duration. One caveat: the
Macaulay formula for duration is correct only if interest rates are
continuously compounded.
Take, for example, a 5-year
zero-coupon note. Because it pays no
coupons, its average maturity is precisely 5 years. Hence, based on the
Macaulay formula for duration, the bond's duration will be 5 years. This
means that a 5-year zero will appreciate about 5% in value for each 1%
decline in continuously compounded interest rates based on approximation [2].
In formula [3], all present values should be
calculated using the
spot interest rate for the maturity of the cash flow
it is discounting. In practice, people often calculate all present values
with a non-continuously compounded yield to maturity 0y
for the entire portfolio. If this is done, formula [3]
must be modified slightly. It becomes
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[4] |
where m is the frequency of compounding for the
yield to maturity. For example, if the yield to maturity is compounded quarterly,
m = 4. This formula is called modified duration.
For portfolios containing instruments that do not pay fixed cash flows, such as callable
bonds, mortgage-backed securities or interest rate caps, the Macaulay or
modified
formulas for duration will not work. For these portfolios, other means
must be employed for calculating duration.
Now let's consider convexity. If duration summarized the most
significant piece of information about a bond or a portfolio's sensitivity
to interest rates, convexity summarizes the second-most significant piece
of information. Duration captured the fact that the graph in
Exhibit 2 was
downward sloping. It did not, however, capture its upward curvature.
Convexity describes curvature.
Exhibit 4 shows the best-fit parabola for the graph of
Exhibit 2:
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Convexity reflects curvature. Here a "best
fit" parabola is fit to the graph of
Exhibit 2. |
Note that the best-fit parabola does not exactly overlay the curve in
Exhibit 4 because the curve is not itself a parabola. In general, the
best-fit parabola will have the form
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[5] |
where convexity is defined as a weighted
second partial derivative
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[6] |
The best fit parabola is simply a second order
Taylor polynomial.
Our first-order approximation [2] now becomes a
second-order approximation:
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[7] |
The thing to remember about convexity is that it is a metric of
curvature. In Exhibit 4, the curvature of the graph bends upward (like a
bowl). The convexity is positive. If the curvature bends downward (like an
inverted bowl), the convexity is negative.
Duration and convexity have traditionally been used as tools for
asset-liability
management. To avoid exposure to parallel spot curve shifts, an
organization (such as an insurance company or defined benefit pension
plan) with significant fixed income exposures might structure its assets
so that their duration matches the duration of its liabilities—so the two
offset. This technique is called duration
matching. Even more effective (but less frequently practical) is
duration-convexity matching,
in which assets are structured so that durations and convexities match.
In closing, it is worth mentioning that terminology associated with the
notion of duration is non-standardized. Different people will use terms in
different ways. This is due to the history of the notion duration.
Macaulay (1938) first introduced the notion of duration as simply weighted
average maturity. To him, "duration" was what we now call Macaulay
duration. Later, people realized that, if Macaulay duration was calculated
using continuously compounded interest rates, the result equaled the
factor sensitivity [1] that is called "duration" in
this article. Because people didn't typically think in terms of
continuously compounded rates, this lead to the modification of Macaulay's
formula, which is now called "modified duration." Other terms, including
"effective duration" and "option-adjusted duration" are also used. As a
rule of thumb, if someone speaks to you about some duration concept, ask
what they mean.
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asset-liability management Techniques for protecting a firm's solvency in the context of accrual accounting.
bond Securitized debt.
compounding Simple, compound and continuously compound interest.
delta and gamma Factor
sensitivities used with derivatives that are analogous to duration
and convexity.
directional strategy
A trading or investment strategy that entails taking net long or short
positions in a market.
financial
risk management Practices by which a firm optimizes the
manner in which it takes financial risk.
fixed income
term structure Refers collectively to a spot curve, forward curve,
discount curve, yield curve or any other curve that describes the time value of
money at a particulate point in time.
gap analysis
A technique of asset-liability management used to assess interest rate risk or
liquidity risk.
Greeks A set of
factor sensitivities used to measure risk exposures related to
options or other derivatives.
interest rate risk
Risk due to uncertain future interest rates.
interest rate
spreads Spreads between two interest rates.
option-adjusted spread
Yield spread not attributable to embedded options.
return Any of a number of metrics for the
change in an asset's or portfolio's accumulated value
scenario analysis
Formalized "what if" analysis typically performed as a part of asset-liability management
or corporate risk management.
yield
Any of several metrics of the income or return to be earned from an investment. |
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Ads by Contingency Analysis
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Frederick R. Macaulay first introduced the notion of duration
in his 1938 book The Movement of Interest Rates, Bond Yields
and Stock Prices in the United States Since 1856. |
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