Therefore, the change in the bond price is not exactly the same as it would be if its own yield-to-maturity changed by the same amount as the change in the par curve. In general, the modified duration and effective duration on a traditional option-free bond are not identical. The difference narrows when the yield curve is flatter, the time-to-maturity is shorter, and the bond is priced closer to par value (so that the difference between the coupon rate and the yield-to-maturity is smaller). The modified duration and effective duration on an option-free bond are identical only in the rare circumstance of an absolutely flat yield curve.
(Institute 540)
Institute, CFA. 2016 CFA Level I Volume 5 Equity and Fixed Income. CFA Institute, 07/2015. VitalBook file.
上面这段话是教材里面的，没看懂，为什么说modified duration and effective duration is identical when the yield curve is flat.
