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ddmmyy · 2021年04月04日

问一道题:NO.PZ2015121810000014 [ CFA II ]

问题如下:

The benchmark portfolio is the S&P 500. Which of the following three portfolios can be combined with the benchmark portfolio to produce the highest combined Sharpe ratio?

选项:

A.

Portfolio A

B.

Portfolio B

C.

Portfolio C

解释:

B is correct.

The active portfolio that is optimal is the portfolio with the highest Information ratio, the ratio of active return to active risk. The IRs for the three active portfolios are:

IRAIR_A= 1.0/10.0 = 0.10

IRBIR_B = 0. 5/3.0 = 0.167

IRCIR_C= 0/2.0 = 0.00

Portfolio B has the highest IR and is the best active portfolio; it is therefore the best portfolio to combine with the benchmark.

考点:Sharpe ratio

解析:SR2=SRB2+IR2SR^2=SR_B^2+IR^2,benchmark的Sharpe ratio对于每个Combined portfolio都相同,因此information ratio最大的组合将会得到最大的Combined Sharpe ratio。

ABC组合的IR分别为:0.1,0.167,0。 组合B的IR最大,因此, combined Sharpe ratio 最大。

讲义里说SR combine等于SR portfolio,为什么不能直接用portfolio A的SR最大来判断?
1 个答案

星星_品职助教 · 2021年04月05日

同学你好,

本题题干中说明“...three portfolios can be combined with the benchmark portfolio...”

所以本题的combined portfolio是主动管理的portfolio和benchmark的组合,不是和Rf的组合。

你列举的式子里:

1)不是本题所描述的combined portfolio

2)这个式子的目的是描述在portfolio里增加/减少cash,portfolio的SR是不变的。和本题不符(并没有提到cash相关)