Why effective duration matters
Modified duration assumes fixed cash flows, which can misstate risk when borrowers or issuers respond to rates. Effective duration incorporates that behavior through a valuation model and is therefore central to mortgage and callable-bond risk management.
How it is applied
The security is valued at its current curve and after equal small downward and upward curve shocks. The difference between shocked prices is divided by twice the initial price and the yield shock. The calculation reprices the instrument after small upward and downward shifts in the benchmark yield curve while allowing expected cash flows to change. This makes it suitable for callable bonds, mortgage-backed securities, and other instruments whose payment timing depends on rates. Portfolio effective duration is usually the market-value-weighted contribution of its holdings.
Portfolio example
A callable bond rises only modestly when yields fall because redemption becomes more likely, but falls more when yields rise. Its effective duration may therefore be lower than the modified duration calculated from promised maturity cash flows. Suppose a bond is worth 100 today, 96.5 after a 1 percentage point rate increase, and 104 after an equivalent decrease. Effective duration is approximately (104 minus 96.5) divided by 2 times 100 times 0.01, or 3.75 years. A smaller rate shock is normally used in professional models.
How to interpret it
A higher effective duration indicates greater modeled price sensitivity to the specified rate shock. The result reflects both discount-rate change and modeled option exercise, so assumptions about volatility and borrower behavior matter. A duration of 3.75 implies an estimated 3.75% price decline for a parallel 1 percentage point yield rise, before convexity. Negative effective duration can occur when expected cash flows shorten as rates rise. Analysts should identify the curve, model, rate shock, and option assumptions behind the reported number.
Limitations and common misconceptions
The measure depends on model, curve, shock size, volatility, and prepayment or call assumptions. It usually assumes a parallel move and omits spread, credit, and liquidity shocks. Results from different models may not be directly comparable. The estimate is local and may be inaccurate for large or nonparallel curve changes. Results depend on prepayment, volatility, and option-exercise models. Credit-spread changes are usually outside the calculation. Comparing effective duration from different vendors can therefore mix different cash-flow assumptions even when the displayed figures look identical.
Sources and further reading
- Fixed-Income Securities: Defining ElementsCFA Institute
- Fixed-Income Bond Valuation: Prices and YieldsCFA Institute
- BondsFINRA