CFA二级重要知识点：OAS、Z-spread和Normal spread区别联系

spread 通常所说的利差，主要有以下几种分类：
1.Normal spread：is the simplest of the spread measures to use and to understand.It is simply an issue's YTM minus the YTM of a Treasure security of similar maturity.
即：普通spread=债券到期收益率-无风险债券到期收益率，
也就是风险观察中常说的利差spread，或者是信用利差。


2.Zero volatility spread (Z-spread): is the equal amount that we must add to each rate on the Treasure spot yield curve in order to make the present value of the risky bond's cash flow equal to its market price.
For a risky bond, the value obtained from discounting the expected cash flows at treasury spot rates will be too high because the treasure spot rate are lower than those appropriate for a risky bond. In order to value it correctly, we have to increase each of the treasure spot rates by some equal amount so that the present value of the risky bond's cash flows discounted at the increased spot rates equals the market value of the bond.
Z-spread 与normal sp