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wilsonxu · 2021年08月21日

老师这个一阶导和二阶导不能混用还可以在详细说明一下吗?

NO.PZ2018122701000018

问题如下:

The bank’s trading book consists of the following two assets:

Correlation (A, B) = 0.2

How would the daily VaR at 99% level change if the bank sells $50 worth of asset A and buys $50 worth of asset B?

Assume there are 250 trading days in a year.

选项:

A.

$0.2286

B.

$0.4776

C.

$0.7705

D.

$0.7798

解释:

B is correct.

考点 Parametric Estimation Approaches

解析 The trade will decrease the VaR by 0.4776。

易错点:求daily VaR,但题干给的是annual return。

t=0,组合由$100A+$50B构成,μP =13.33%, 由年转化为天,13.33%/250=0.0533%.

σP =19.15%, 由年转化为天, 19.15%/SQRT(250)=1.2111%

daily 99%VaR=(2.33*1.2111%-0.0533%)*$150=$4.1528

t=1,组合由$50A+$100B构成,μP =16.66%, 由年转化为天,16.66%/250=0.0667%.

σP =17.08%, 由年转化为天, 17.08%/SQRT(250)=1.0801%

daily 99%VaR=(2.33*1.0801%-0.0667%)*$150=$3.6749

所以$4.1528-$3.6749=0.4779,最接近的是B选项。

不能,有μ的时候不能这么算。 先求俩var,再算组合是建立在假设了μ等于0的基础上的,有了μ的话两个var求组合var的那个公式就不成立了,因为一阶导和二阶导不能混用。
AliciaLi · 2022年01月18日

σP =19.15%,这里怎么算出来的, 能详细列一下计算吗, 我用了Varp^2=(W1^2 * Var1^2)+(W2^2 * Var2^2)+0.2*W1*W2*Var1*Var2 但是得不到正确的数字

2 个答案
已采纳答案

李坏_品职助教 · 2021年08月21日

嗨,努力学习的PZer你好:


当μ不为0的时候,我们就必须先求资产组合的σ,再求组合的var。因为这个时候μ会影响资产组合的μ,如果直接把两个资产的var组合起来会不准确。


这里应该和导数没有什么关系。



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李坏_品职助教 · 2022年01月18日

嗨,爱思考的PZer你好:


表格里的volatility指的是标准差。σa = 0.25, σb = 0.2. 因为题目设定A是100,B是50,所以Wa = 100/150 = 2/3,Wb = 1/3


σp^2 = ((2/3)^2 * 0.25^2 ) + ((1/3)^2 * 0.2^2) + 2*(2/3)* (1/3) * correlation * 0.25 * 0.2 =  0.03667. 所以σp = 0.1915

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