Effective Duration vs. Modified Duration: What's the Difference?
Not every bond’s future cash flows are fixed and predictable. Once an issuer has the option to call a bond early, or a borrower has the option to prepay, the standard way of measuring interest rate sensitivity needs an adjustment.
The short answer
Modified duration estimates a bond’s price sensitivity to rate changes assuming its cash flows stay fixed no matter what rates do, while effective duration accounts for the possibility that cash flows themselves might change, such as when a bond can be called early. Effective duration is the more appropriate measure for bonds with embedded options; modified duration works well for plain, option-free bonds.
How modified duration is built
Modified duration starts from a bond’s duration and adjusts it to estimate the percentage price change for a given change in yield, under the assumption that the bond’s future coupon and principal payments won’t change regardless of where rates go. For a straightforward bond — one with no call feature, no prepayment option, nothing but a fixed schedule of payments — that assumption holds up well, and modified duration gives a reliable estimate of price sensitivity.
Where that assumption breaks down
Plenty of bonds don’t have cash flows that are truly locked in. A callable corporate bond gives the issuer the right to redeem the bond before maturity, typically when rates have fallen enough that refinancing at a lower cost makes sense for the issuer. Mortgage-backed securities carry a related feature, since underlying homeowners can prepay their loans, especially when refinancing becomes attractive. In both cases, the cash flow schedule an investor assumed at purchase might not hold if rates move enough to trigger early repayment.
How effective duration adjusts for this
Effective duration is calculated differently: instead of assuming fixed cash flows, it estimates how a bond’s price would actually respond to small changes in rates by modeling the cash flows under a few different rate scenarios, including the possibility that an option gets exercised. This produces a duration figure that reflects the real-world behavior of the bond, including the way a call feature can cap how much a bond’s price rises when rates fall.
A simple way to think about the difference
- Modified duration. Assumes cash flows are locked in; works well for option-free bonds like most government notes.
- Effective duration. Accounts for cash flows that could shift; more appropriate for callable bonds and mortgage-backed securities.
- The gap between them. Widens as a bond’s embedded option becomes more likely to be exercised, such as when rates have fallen well below the bond’s coupon rate.
Why the distinction matters when comparing bonds
Using modified duration on a callable bond can overstate how much its price would rise if rates fell sharply, because it ignores the likelihood the issuer would call the bond and cap that gain. This connects closely to the idea of convexity, since callable bonds often exhibit different curvature than option-free bonds precisely because of this capped upside. Comparing bonds by duration and maturity together, while also noting whether either measure is modified or effective, avoids drawing the wrong conclusion about relative interest rate risk.
What to weigh
For a bond without embedded options, modified duration is usually sufficient and simpler to work with. For anything with a call feature, a prepayment option, or another structural quirk that could change its cash flows, effective duration gives a more realistic sense of how the bond’s price is likely to actually behave when rates move.