问题如下图：
选项：
A. 
B. 
C. 
解释：
这道题问题是什么意思呢？做题的时候知道考点是什么，但是联系到题目就不知道怎么作答了。
菲菲_品职助教 · 2018年11月08日
同学你好，这题是问，在B的三个结论中，哪一条既能满足random walk的性质，又能满足协方差平稳的性质。
Conclusion 1: The variance of xt increases over time.
这一条就违反了协方差平稳条件的第二条：The variance of the time series must be constant and finite in all periods.
Conclusion 2: The mean-reverting level is undefined.
这一条说均值复归水平是不确定的，但对于协方差平稳的时间序列来说，均值复归水平是确定的，所以这一个结论也不符合条件。
Conclusion 3: b0 does not appear to be significantly different from 0.
这个结论翻译过来的意思就是b0=0。我们知道random walk的b0=0，所以符合random walk。此外，协方差平稳的时间序列里面也没有条件或者性质规定b0的取值，所以b0=0也符合协方差平稳的时间序列。综上所述，结论3是我们应该要选取的选项。
这道题目考的比较综合，结合了两个知识点，所以通过原版书的这几个case题，要学会培养自己的做题思路，有些题目并不像一级考试中那么容易理解，所以更要学会提炼题干和问题的关键信息。
NO.PZ201709270100000501 问题如下 1.Whiof Busse’s conclusions regarng the exchange rate time series is consistent with both the properties of a covariance-stationary time series anthe properties of a ranm walk? A.Conclusion 1 B.Conclusion 2 C.Conclusion 3 C is correct. A ranm walk cscribethe equation xt = + b1xt–1+ εt, where = 0 an= 1. So = 0 is a characteristic of a ranm walk time series. A covariance-stationary series must satisfy the following three requirements:1. The expectevalue of the time series must constant anfinite in all perio.2. The varianof the time series must constant anfinite in all perio.3. The covarianof the time series with itself for a fixenumber of perio in the past or future must constant anfinite in all perio.= 0 es not violate any of these three requirements anis thus consistent with the properties of a covariance-stationary time series. 不违反ranm walk的性质 但是ranm walk性质就是b=0啊
NO.PZ201709270100000501问题如下1.Whiof Busse’s conclusions regarng the exchange rate time series is consistent with both the properties of a covariance-stationary time series anthe properties of a ranm walk? A.Conclusion 1B.Conclusion 2C.Conclusion 3C is correct. A ranm walk cscribethe equation xt = + b1xt–1+ εt, where = 0 an= 1. So = 0 is a characteristic of a ranm walk time series. A covariance-stationary series must satisfy the following three requirements:1. The expectevalue of the time series must constant anfinite in all perio.2. The varianof the time series must constant anfinite in all perio.3. The covarianof the time series with itself for a fixenumber of perio in the past or future must constant anfinite in all perio.= 0 es not violate any of these three requirements anis thus consistent with the properties of a covariance-stationary time series. 请问covariance-stationary和ranm walk是对立关系吗？
NO.PZ201709270100000501 老师，提问区其他的提问跟这个问题不符。另外，可以一下B为啥不对吗？C不是答案刚开始就说不正确吗，并没有说with ift啊
NO.PZ201709270100000501 这个知识点可以一下吗
Conclusion 2 Conclusion 3 C is correct. A ranm walk cscribethe equation xt = + b1xt–1+ εt, where = 0 an= 1. So = 0 is a characteristic of a ranm walk time series. A covariance-stationary series must satisfy the following three requirements: 1. The expectevalue of the time series must constant anfinite in all perio. 2. The varianof the time series must constant anfinite in all perio. 3. The covarianof the time series with itself for a fixenumber of perio in the past or future must constant anfinite in all perio. = 0 es not violate any of these three requirements anis thus consistent with the properties of a covariance-stationary time series. 看了前面人问的问题，有一点想确认下，ranm walk是不是包含ranm walk with a ift？如果不是，那ranm walk和simple ranm walk就是一回事？
