Bond duration

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In finance, duration refers to two concepts that seem unrelated, but turn out to be very closely related:

  • the weighted average term (time from now to payment) of a bond's cash flows or of any series of linked cash flows.
  • the sensitivity of a bond's theoretically correct price to interest rate (ie yield) movements.

Thus the duration of a zero coupon bond with a maturity period of n years is n years. If there are coupon payments, the duration will be less than n years.

This article contains a lot of advanced material, and the above statement will be sufficient for many readers.

This measure is closely related to the derivative of the bond's price function with respect to the interest rate (in terms of the Greeks, this is referred to as the Δ or Delta, where the underlying is the interest rate), and some authors define the duration to be this derivative divided by the price (in terms of the Greeks, this ratio is referred to as the λ or Lambda). This ratio is the weighted average term, with appropriate weightings for a non-callable bond.

The duration is often confused with other notions, as detailed below.

Contents

[edit] Price

Duration is useful primarily as a measure of the sensitivity of a bond's market price to interest rate (ie yield) movements. It is approximately equal to the percentage change in price for a given change in yield. For example, for small interest rate changes, the duration is the approximate percentage by which the value of the bond will fall for a 1% per annum increase in market interest rate. So a 15-year bond with a duration of 7 would fall approximately 7% in value if the interest rate increased by 1% per annum. [1] In other words, duration is the elasticity of the bond's price with respect to interest rates.

[edit] Basics

The general standard definition of duration:

D = \sum_{i=1}^{n}\frac {P(i)t(i)}{V}

where P(i) is the present value of coupon i or the final principal payment, t(i) is the time from now until the payment date, V is the bond price and D is the duration.

[edit] Cash flow

As stated above, the duration is the weighted average term to payment of the cash flows on a bond. For a zero-coupon the duration will be ΔT = Tf − T0, where Tf is the maturity date and T0 is the starting date of the bond. If there are additional cash flows Ci at times Ti, the duration of every cash flow is ΔTi = Ti − T0. From the current market price of the bond V, one can calculate the yield to maturity of the bond r using the formula

V = \sum_i P(i) = \sum_i C_i e^{-r\Delta T_i}.

Note that in this and subsequent formulae, the symbol r is used for the force of interest, ie the logarithm of (1+j) where j is the interest yield expressed as an annual effective yield.

In a standard duration calculation, the overall yield of the bond is used to discount each cash flow leading to this expression in which the sum of the weights is 1:

D = \sum_i \Delta T_i \frac{C_i e^{-r\Delta T_i}}{V}

The higher the coupon rate of a bond, the shorter the duration (if the term of the bond is kept constant). Duration is always less than or equal to the overall life (to maturity) of the bond. Only a zero coupon bond (a bond with no coupons) will have duration equal to the maturity.

Duration indicates also how much the value V of the bond changes in relation to a small change of the rate of the bond. We see that

\frac{\partial V}{\partial r} = - \sum_i \Delta T_i C_i e^{-r\Delta T_i} = -D \cdot V

so that for a small variation \partial r in the redemption yield of the bond we have

\frac{\partial V}{V} = -D \partial r + O(\partial r^2)

That means that the duration gives the negative of the relative variation of the value of a bond with respect to a variation in the redemption yield on the bond, forgetting the quadratic and higher-order terms. The quadratic terms are taken into account in the Convexity.

As we have seen above, r=ln(1+j).

If \frac{\partial V}{\partial j} (which could be defined as the Modified Duration) is required, then it is given by:

 \frac{\partial V}{\partial j} = \frac{\partial V}{\partial r}. \frac{dr}{dj}

=(-D.V) .d(ln(1+j))/dj

=(-D.V)/(1+j)

and this relationship holds good whatever the frequency of convertibility of j.

[edit] Dollar duration and applications to VaR

The Dollar duration is defined as the product of the Duration and the price (value). It gives the variation in a bond's value for a small variation in the yield. Dollar duration D$ is commonly used for VaR (Value-at-Risk) calculation. If V = V(r) denotes the value of a security depending on the interest rate r, dollar duration can be defined as
D_$ := -\frac{\partial V}{\partial r} .

To illustrate applications to portfolio risk management, consider a portfolio of securities dependent on the interest rates r_1, \ldots, r_n as risk factors, and let
V = V(r_1, \ldots, r_n)
denote the value of such portfolio. Then the exposure vector \boldsymbol{\omega} = (\omega_1, \ldots, \omega_n) has components
\omega_i = - D_{$,i} := \frac{\partial V}{\partial r_i}
Accordingly, the change in value of the portfolio can be approximated as
\Delta V = \sum_{i=1}^n \omega_i \Delta r_i + \sum_{1 \leq i,j \leq n} O(\Delta r_i \Delta r_j)
that is, a component that is linear in the interest rate changes plus an error term which is at least quadratic. This formula can be used to calculate the VaR of the portfolio by ignoring higher order terms. Typically cubic or higher terms are truncated. Quadratic terms, when included, can be expressed in terms of (multi-variate) bond convexity. One can make assumptions about the joint distribution of the interest rates and then calculate VaR by Monte Carlo simulation or, in some special cases (e.g., Gaussian distribution assuming a linear approximation), even analytically. The formula can also be used to calculate the DV01 of the portfolio (cf. below) and it can be generalized to include risk factors beyond interest rates.

[edit] Macaulay duration

Macaulay duration, named for Frederick Macaulay who introduced the concept, is the weighted average maturity of a bond where the weights are the relative discounted cash flows in each period.

\mbox{Macaulay duration} = \frac {\sum\ (\mbox{cash flow discounted with yield to maturity}\times\mbox{time to cash flow})}{\mbox{price of the bond}}.

It will be seen that this is the same formula for the duration as given above.

Macaulay showed that an unweighted average maturity is not useful in predicting interest rate risk. He gave two alternative measures that are useful:

  • The theoretically correct Macaulay-Weil duration which uses zero-coupon bond prices as discount factors, and
  • the more practical form (shown above) which uses the bond's yield to maturity to calculate discount factors.

The key difference between the two is that the Macaulay-Weil duration allows for the possibility of a sloping yield curve, whereas the algebra above is based on a constant value of r, the yield, not varying by term to payment.

With the use of computers, both forms may be calculated, but the Macaulay duration is still widely used.

In case of continuously compounded yield the Macaulay duration coincides with the opposite of the partial derivative of the price of the bond with respect to the yield—as shown above. In case of yearly compounded yield, the modified duration coincides with the latter.


[edit] Embedded options and effective duration

For bonds that have embedded options, such as puttable and callable bonds, Macaulay duration will not correctly approximate the price move for a change in yield.

In order to price such bonds, one must use option pricing to determine the value of the bond, and then one can compute its delta and hence its lambda), which is the duration. The effective duration is a discrete approximation to this latter, and depends on an option pricing model.

Consider a bond with an embedded put option. As an example, a $1,000 bond that can be redeemed by the holder at par at any time before the bond's maturity (ie an American put option). No matter how high interest rates become, the price of the bond will never go below $1,000 (ignoring counterparty risk). This bond's price sensitivity to interest rate changes is different from a non-puttable bond with otherwise identical cashflows. Bonds that have embedded options can be analyzed using "effective duration". Effective duration is a discrete approximation of the slope of the bond's value as a function of the interest rate.

\text{Effective Duration} = \frac {V_{-\Delta y}-V_{+\Delta y}}{2(V_0)\Delta y}

where Δy is the amount that yield changes, and V − ΔyandV + Δy are the values that the bond will take if the yield falls by y or rises by y, respectively. However this value will vary depending on the value used for Δy.

[edit] Average duration

The sensitivity of a portfolio of bonds such as a bond mutual fund to changes in interest rates can also be important. The average duration of the bonds in the portfolio is often reported. The duration of a portfolio equals the weighted average maturity of all of the cash flows in the portfolio. If each bond has the same yield to maturity, this equals the weighted average of the portfolio's bond's durations. Otherwise the weighted average of the bond's durations is just a good approximation, but it can still be used to infer how the value of the portfolio would change in response to changes in interest rates.

[edit] Bond duration closed-form formula

Dur=\frac{C}{P}\frac{(1+ai)(1+i)^m-(1+i)-(m-1+a)i}{i^2(1+i)^{(m-1+a)}}+\frac{100(m-1+a)}{(1+i)^{(m-1+a)}}


C = coupon payment per period (half-year)
i = discount rate per period (half-year)
a = fraction of a period remaining until next coupon payment
m = number of coupon dates until maturity
P = bond price

[edit] Convexity

Main article: Bond convexity

Duration is a linear measure of how the price of a bond changes in response to interest rate changes. As interest rates change, the price does not change linearly, but rather is a convex function of interest rates. Convexity is a measure of the curvature of how the price of a bond changes as the interest rate changes. Specifically, duration can be formulated as the first derivative of the price function of the bond with respect to the interest rate in question, and the convexity as the second derivative.

Convexity also gives an idea of the spread of future cashflows. (Just as the duration gives the discounted mean term, so convexity can be used to calculate the discounted standard deviation, say, of return.)

[edit] PV01 and DV01

PV01 is the present value impact of 1 basis point move in an interest rate. It is often used as a price alternative to duration (a time measure). It is also known as DV01 (Dollar Value of 1 basis point.

[edit] Confused notions

Duration, in addition to having several definitions, is often confused with other notions, particularly various properties of bonds that are measured in years.

Duration is sometimes explained inaccurately as being a measurement of how long, in years, it takes for the price of a bond to be repaid by its internal cash flows. This quantity is simply \frac{1}{r}, assuming the tenor is this long, or the tenor otherwise (for instance, if a bond pays 5% per annum and was issued at par, it will take 20 years of these payments to repay its price), and is the duration of a perpetual bond, assuming a flat yield curve at the coupon. Note the absurdity of this definition: given a bond paying 5% per annum with a tenor of 5 years, the duration will be approximately 2.5, while the price of the bond will not be repaid in full until maturity (at 5 years).

The Weighted-Average Life is the weighted average of the principal repayments of an amortizing loan, and is longer than the duration.

[edit] See also

[edit] Lists

[edit] References

  1. ^ "Macaulay Duration" by Fiona Maclachlan, The Wolfram Demonstrations Project.

[edit] External links

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