弱弱的问一个关于effective duration的问题 option, 价格变化, effective

jaywang0123

为什么effective duration要用benchmark yield curve的变化来考虑有含有option的bond的价格变化？

jaywang0123

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.