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RyanR · 2023年10月26日

什么叫risk a Type I error?

NO.PZ2018122701000033

问题如下:

Basel II requires a backtest of a bank’s internal value at risk (VaR) model (IMA). Assume the bank’s ten-day 99% VaR is $1 million (minimum of 99% is hard-wired per Basel). The null hypothesis is: the VaR model is accurate. Out of 1,000 observations, 25 exceptions are observed (we saw the actual loss exceed the VaR 25 out of 1000 observations).  (Binomial CDF)

选项:

A.

We will probably call the VaR model good (accurate) but we risk a Type I error.

B.

We will probably call the VaR model good (accurate) but we risk a Type II error.

C.

We will probably call the model bad (inaccurate) but we risk a Type I error.

D.

We will probably call the model bad (inaccurate) but we risk a Type II error.

解释:

C is correct.

考点 : Backtesting VaR

解析 :H0 : the VaR model is accurate. Hα: the VaR model is inaccurate.

Z=xpTp(1p)T=251%×10001%×(11%)×1000=4.77Z=\frac{x-pT}{\sqrt{p(1-p)T}}=\frac{25-1\%\times1000}{\sqrt{1\%\times(1-1\%)\times1000}}=4.77

As 4.77 is larger than 2.58, we reject the null hypothesis. Therefore, the model is bad model, and this implies a risk of type I error.

大概能懂题意,我们对银行的一个模型进行backtesing, 然后银行的模型给的VaR是99%,而我们实际测出来的是1000次25个exception。所以我们的结论是这个模型不准,但是因为原假设H0是模型是准确的,而我们的结果拒绝了原假设,所以我们犯了一类错误?


有点懵了,那我们既然犯错误了,那对于这个model不准确的结论成立吗?或者说这个犯一类错误的意义是啥,有点没懂。不知道表达清楚没有。

2 个答案

pzqa27 · 2023年11月08日

嗨,爱思考的PZer你好:


这个题的意思很简单,本来银行搞了一个VaR模型,本来根据银行的模型,只能有10天击穿VaR,现在有25天击穿了VaR,那么我们要看一下,这25天跟10天比是否是显著的,也就是说,本来应该有10天击穿VaR,假如说真实的击穿天数是11天,我们还可能认为多出的一天属于合理的误差范围内,现在有25天击穿了,多出了15天,根据假设检验,25天显然是无法近似于10天的,因此我们判断这个模型不算个好模型,相当于我们拒绝了这个模型,因此我们拒真的可能性,就是说这个模型可能是好的,但是我们判断错了。

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就算太阳没有迎着我们而来,我们正在朝着它而去,加油!

pzqa27 · 2023年10月26日

嗨,努力学习的PZer你好:


这里是在做假设检验,相当于断案,一类错误就是说,这个原假设本来是个好人,我们断案断错了,把好人给杀了。

1类错误的概率就是我们错杀好人的概率。

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就算太阳没有迎着我们而来,我们正在朝着它而去,加油!

RyanR · 2023年11月08日

您举的例子我能理解。但是现在这个模型只是假设是准确的,事实上并不一定就是正确的啊。如果这个模型是准确的,而我们拒绝了,那我可以理解是犯了一类错误。 但现在只是判断这个假设对不对而已,那我只要结论和假设不一样,就算犯一类错误了?

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