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为了求职冲呀 · 2020年03月09日

问一道题：NO.PZ2018062016000091

  1. CFA I
  2. Quantitative

问题如下：

Given a discrete random variable Y, the set of all the possible Y value is Y = {20,21,22,23,24,25}.

The estimated value of cumulative distribution function is in the table, what is the probability that Y takes 23 or 25?

选项：

A.

0.50

B.

0.45

C.

0.75

解释：

A is correct. 

F(23) = P(Y≤ 23) = p(20) + p(21) + p(22) + p(23) = 0.55. F(22) = P(Y≤ 22) = p(20) + p(21) + p(22) = 0.30. So p(23) = 0.55-0.30 = 0.25.

For the same reason, F(25) = 1.00, F(24) = 0.75, p(25) = 1.00-0.75 = 0.25.

The probability that Y takes 23 or 25 = p(23) + p(25) = 0.25+0.25 = 0.50

老师好，根据cumulative p23不应该是0.08+0.3+0.2=0.58吗？

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1 个答案

星星_品职助教 · 2020年03月09日

同学你好，

这道题里的P(23)指的是X=23这件事发生的概率，或者说写成P(X=23)，并不是累积概率，你写的是F(23)的公式。

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