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徐威廉 · 2020年07月03日

问一道题:NO.PZ201601200500000801

* 问题详情,请 查看题干

问题如下:

1. What is the optimal service life for Laroche Livery’s fleet of vans?

选项:

A.

Two years.

B.

Three years.

C.

Four years.

解释:

B is correct.

The way to solve the problem is to calculate the equivalent annual annuity and choose the service life with the lowest annual cost. For a two-year service life, the NPV is

NPV=40000+120001.101+150001.102+200001.102=46776.86NPV=-40000+\frac{-12000}{1.10^1}+\frac{-15000}{1.10^2}+\frac{20000}{1.10^2}=-46776.86

The EAA (PV = –46,776.86, N = 2, and i = 10%) is –26,952.38.

For a three-year service life, the NPV is

NPV=40000+120001.101+150001.102+200001.103+170001.103\mathrm{NPV}=-40000+\frac{-12000}{1.10^1}+\frac{-15000}{1.10^2}+\frac{-20000}{1.10^3}+\frac{17000}{1.10^3}

=-65559.73

The EAA (PV = –65,559.73, N = 3, and i = 10%) is –26,362.54.

For a four-year service life, the NPV is

NPV=40000+120001.101+150001.102+200001.103+250001.104+120001.104\mathrm{NPV}=-40000+\frac{-12000}{1.10^1}+\frac{-15000}{1.10^2}+\frac{-20000}{1.10^3}+\frac{-25000}{1.10^4}+\frac{12000}{1.10^4}

=-87211.26

The EAA (PV = –87,211.26, N = 4, and i = 10%) is –27,512.61.

The three-year service life has the lowest annual cost. Laroche should replace the vans every three years.

之前做的题老师讲EAA法不考虑PMT符号,本题为什么将PMT的负号考虑在内?是因为表格内每期现金流都为负号即流出成本吗?到底如何判断EAA法PMT什么时候考虑符号?

1 个答案
已采纳答案

王琛_品职助教 · 2020年07月04日

同学你好,

一. EAA 的原理/本质

李老师在课上,经常和我们分享建议:学习知识,建议重点关注原理,而不是细节。因为很多细节和结论,都可以通过原理推导出来。
所以这里,我们暂时将符号的计算细节放置一旁,先问一个最根本的问题:EAA 的 原理/本质 到底是什么?


我眼中的 EAA:不管项目期限多长,期间现金流是怎样,可以先计算项目的 NPV。为了在 T=0 时刻,有 NPV 这样的现金流,等价于项目期间,每年固定的现金流是多少?
我拿这道题,表格第一行的数据为例,画一下 EAA 的思路。


这里体现出了 EAA 的本质:和 NPV 的符号是相同的。

  • 如果 NPV > 0,代表 T=0 时刻现金「流入」,等价的项目期间固定现金流,肯定是也是「流入」,所以 EAA > 0
  • 如果 NPV < 0,代表 T=0 时刻现金「流出」,等价的项目期间固定现金流,肯定是也是「流出」,所以 EAA < 0

以上我们解释清楚了 EAA 的 原理/本质,下面再说一下计算细节。

二. 计算细节

现在我们计算出了两年项目的 NPV 为 -46776.86,那么 EAA 怎么算呢?
其实最符合我们计算逻辑的,是计算器的第二行:(标有 CF,NPV,IRR 的那一行),但是这一行的使用缺点是,只能根据期间现金流计算 NPV,没法根据 NPV 和 期间现金流数量,反推出固定的期间现金流。


所以,在这种条件下,我们才「迫不得已」使用计算器的第三行(标有 N,I/Y,PV,PMT,FV 的那一行)「间接」得出我们想要的数据。
使用「迫不得已」、「间接」,是因为因为这一行计算器的默认逻辑,和我们这里求 EAA 的逻辑不一样,我们需要在计算时做一些调整,而这个调整才涉及到符号的细节。


所以下面我们自然地引出三个小问题

  • 计算器第三行的计算逻辑是什么?
  • 使用第三行求 EAA,需要做什么调整?
  • 得出 EAA 以后,如何检验结果?

1. 计算器第三行的计算逻辑

在使用计算器第三行的时候,你会发现:PV 和 PMT 的符号是相反的。
计算器第三行的逻辑是:

  • 如果 PV < 0, 相当于你期初一次性投入(现金流出),按照一定的折现率,你期间每年可以获得固定的现金流入是多少,所以 PMT > 0
  • 如果 PV > 0, 相等于你期间每年现金流出,PMT < 0, 按照一定的折现率, 等价于你期初一次性获得多少(现金流入)

2. 使用第三行求 EAA,需要做什么调整?

因为咱们求 EAA,本质是要得到和 NPV 符号相同的数据,所以在使用第三行的时候,要做调整:将 NPV 代入 PV 这一步时,需要代入 -NPV。
比如这道题,两年期项目的 NPV 为 -46776.86 < 0, 你使用计算器时,PV 的取值,一定是 -NPV,即 46776.86 > 0。
只有这样,得出的 PMT 才是 EAA,即 -26952.38 < 0, 和 NPV 符号相同。

3. 得出 EAA 以后,如何检验结果?

如果你担心考试时记不清 EAA 计算时符号的细节问题,没有关系,我们还有一个办法,可以验证你计算出来的结果。
假如你计算时,PV 没有取 -NPV,计算出来的结果是 26952.38 > 0, 那么如何验证这个结果是否是正确的 EAA 呢?


想必你已经有思路了,就是将这个结果代入计算器的第二行,因为只有第二行的计算逻辑才和咱们求 EAA 的逻辑相同。
按计算器第二行的参数:

  • CF0 = 0
  • C01 = 26952.38 > 0
  • F01 = 2
  • I = 10
  • NPV = 46676.86 > 0

相当于期间有两笔相等的现金流,每笔都是 26952.38 > 0, 折现到 T=0 时刻的 NPV 为 46676.86 > 0, 这和之前算出来的两年期项目的 NPV 符号相反。
所以通过验证,正确的 EAA 应该小于 0, 即 -26952.38.

希望通过上面的原理和计算细节的介绍,能够帮你解答这里的疑问。

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2022-11-02 09:24 3 · 回答

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2022-06-14 02:21 1 · 回答

NO.PZ201601200500000801 计算EAA时,都默认FV=0吗?

2022-02-09 16:00 1 · 回答

NO.PZ201601200500000801 你好 这个题我按答案按的计算器 但是结果都是正的 想问下问题出在哪里?

2021-11-30 10:18 2 · 回答

NO.PZ201601200500000801 老师好 这题 是不是计算器里PV 应该打入正的 不是负的 (比如正的46,776.86), 因为 NPV 和E的符号一致。 PV 键和PMT 在计算器第三例 是符号相反, 如果这题里输入负的NPV, 算出的PMT 或EAA会是正的 而不是负的 (cost). 谢谢。

2021-11-25 08:07 1 · 回答