收益率曲线平行移动时，spot rate 变动率 等于 cashflow yield 变动率吗？为什么？

CFY实际上是IRR的一个概念，详见基础班讲义第83页

基础班讲义还有这一段话 （P87）

---------------------------------------------------------------------------------

A sufficient, but not necessary, condition for immunization is a parallel (or s preserving) shift whereby all yields change by the same amount in the same direction.

• Sufficient means that if the yield curve shift is parallel, the change in the bond portfolio’s cash flow yield will equal the change in yield to maturity of the zero-coupon bond, which is enough to ensure immunization.

• To achieve immunization, however, it is not necessary that the yield curve shifts in a parallel manner. If the change in the cash flow yield is the same as that on the zero-coupon bond being replicated, immunization can be achieved even with a non-parallel shift to the yield curve.

CFY对标的是YTM，正因为CFY是IRR的一个概念，他需要知道FY,I/Y,N,Coupon，才能算出来. YTM也是假设债券持有到期的情况下，算出来的。（0息债 spot rate就是YTM）。

收益率曲线平行移动，比如是收益率曲线向上平移1个bp，但不代表每个时间点的spot rate的变动率相同（假设1年期的spot rate 为1%，2年的为2%，3年的为3%）。那么1bp对于1%，2%，3%来说的变动率肯定是不一样的。