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FrankSun · 2019年12月01日

问一道题：NO.PZ2017092702000073

  1. CFA I
  2. Quantitative

问题如下：

The probability distribution for a company’s sales is:

The standard deviation of sales is closest to:

选项：

A.

$9.81 million.

B.

$12.20 million.

C.

$32.40 million.

解释：

A is correct.

The analyst must first calculate expected sales as 0.05 × $70 + 0.70 × $40 + 0.25 × $25 = $3.50 million + $28.00 million + $6.25 million = $37.75 million. After calculating expected sales, we can calculate the variance of sales: = σ2 (Sales) = P($70)[$70 – E(Sales)]2 + P($40)[$40 – E(Sales)]2 + P($25)[$25 – E(Sales)]2 = 0.05($70 – 37.75)2 + 0.70($40 – 37.75)2 + 0.25($25 – 37.75)2 = $52.00 million + $3.54 million + $40.64 million = $96.18 million. The standard deviation of sales is thus σ = ($96.18)1/2 = $9.81 million.

2ND - 7 - X01=70; Y01=5 - X02=40; Y02=70 - X03=25; Y03=25 - 2ND - 8 - 2ND - ENTER(重复按这一步，直到变成1-V模式 - 往下翻 - 找到σx就是题目要我们求得标准差。


为什么算出来是18.7呢

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星星_品职助教 · 2019年12月01日

同学你好，

这个题是加权平均的形式，也就是根据概率算方差（然后再开方得到标准差）。只有算术平均的前提下才能用计算器。

所以这道题需要代方差的公式，然后算加权平均的，也就是答案解析的那种算法，括号后面的“2”是平方哈。可以自己手算一下，这种题属于比较烦但是不难的题，加油。

絮飛W涙 · 2021年04月27日

老师计算器1—v是啥意思为什么选到这个模式才可以

星星_品职助教 · 2021年04月29日

@絮飛W涙

这个的原理没有讲过，不是上课内容，固定模式，只能记忆一下了

星星_品职助教 · 2021年04月28日

@絮飛W涙

同学你好，这种计算器的模式上课没有讲过，也不要求掌握，是固定模式，如果觉得这样更方便的话直接记忆过程就可以了。

絮飛W涙 · 2021年04月29日

感觉特别方便，记是记住了，但是就想知道每个按钮都有什么区别，为什么1-v就可以，知道原理可能会记忆更深刻

絮飛W涙 · 2021年04月27日

老师计算器1—v是啥意思为什么选到这个模式才可以

郭大路 · 2019年12月26日

你按错了。

FrankSun · 2020年02月20日

我又算了一次：2ND - 7 - X01=70; Y01=5 - X02=40; Y02=70 - X03=25; Y03=25 - 2ND - 8 - 2ND - ENTER，σx还是18.708287。为什么呢？我计算器是坏的？

星星_品职助教 · 2020年02月20日

如果要这么做，步骤是：2ND - 7 - X01=70; Y01=5 - X02=40; Y02=70 - X03=25; Y03=25 - 2ND - 8 - 2ND - ENTER(重复按这一步，直到变成1-V模式 - 往下翻 - 找到σx就是题目要我们求得标准差。

FrankSun · 2020年02月25日

明白了，之前没有操作2ND - ENTER(重复按这一步，直到变成1-V模式）这一步。谢谢！

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