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赵小丫 · 2021年04月04日

陷在minimize思路里了,求老师解答

NO.PZ2017092702000088

问题如下:

A portfolio has an expected return of 7% with a standard deviation of 13%. For an investor with a minimum annual return target of 4%, the probability that the portfolio return will fail to meet the target is closest to:

选项:

A.

33%.

B.

41%.

C.

59%

解释:

B is correct.

There are three steps, which involve standardizing the portfolio return: First, subtract the portfolio mean return from each side of the inequality: P(Portfolio return – 7%) ≤ 4% – 7%). Second, divide each side of the inequality by the standard deviation of portfolio return: P[(Portfolio return – 7%)/13% ≤ (4% – 7%)/13%] = P(Z ≤ –0.2308) = N(–0.2308). Third, recognize that on the left-hand side we have a standard normal variable, denoted by Z and N(–x) = 1 – N(x). Rounding –0.2308 to –0.23 for use with the cumulative distribution function (cdf) table, we have N(–0.23) = 1 – N(0.23) = 1 – 0.5910 = 0.409, approximately 41 percent. The probability that the portfolio will underperform the target is about 41 percent.

老师,看到这道题我第一思路是:E(Rp)=7%,方差=13%,Rl=4%,可求出SFR=23%。题目问 target fali,我理解意思是寻找SFR>23%的数字,因为这样Rl就会<4%,没有达到minimize的要求,我陷在这个思路里出不来了,没有想到用标准化公式。请老师帮忙指点迷津,谢谢!

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星星_品职助教 · 2021年04月06日

同学你好,

从题干中的“the probability that the portfolio return will fail to meet the target”可以看出,求的直接就是P(X<4%)的概率。

接下来的思路是由于这个概率没法直接查表得到,所以只有先做标准化后才能查表。然后代入标准化的公式就可以了。

整个过程中,不需要涉及到SFR。如果从SFR的角度出发,由于数据都存在,那么算出来的SFR就是23%了。此时不存在其他的SFR。也就不存在另外一个SFR。此时SFR无论是等于59%或41%或33%都是不存在的。

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