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WINWIN8 · 2022年07月10日

这道题是在玩文字游戏吗?

* 问题详情,请 查看题干

NO.PZ201709270100000404

问题如下:

4. Martinez’s Conclusion 1 is:

选项:

A.

correct.

B.

incorrect because the mean and variance of WTI oil prices are not constant over time.

C.

incorrect because the Durbin–Watson statistic of the AR(2) model is greater than 1.75.

解释:

B is correct. There are three requirements for a time series to be covariance stationary. First, the expected value of the time series must be constant and finite in all periods. Second, the variance of the time series must be constant and finite in all periods. Third, the covariance of the time series with itself for a fixed number of periods in the past or future must be constant and finite in all periods. Martinez concludes that the mean and variance of the time series of WTI oil prices are not constant over time. Therefore, the time series is not covariance stationary.

这题开头确实是说了mean and covariance is not constant, 然后他开始做model来验证。

然后一堆验证后得出一个新的结论 “mean and covariance is constant ” 但是答案居然说因为他开头说了“mean and covariance is not constant” 所以得选b。

所以请问这道题有更严密的逻辑吗?

1 个答案

星星_品职助教 · 2022年07月11日

同学你好,

这道题看到题干中的“mean and variance.... is not constant....” 就可以直接选B了。

答案解析里列举的是covariance stationary成立的三个前提条件,并不是去验证 “mean and covariance is constant ”  。正是因为本题开头就提示违反了三个条件中的两个,所以covariance stationary才不成立。

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NO.PZ201709270100000404问题如下 4. Martinez’s Conclusion 1 is: correct. incorrebecause the meanvarianof WTI oil prices are not constant over time. C.incorrebecause the rbin–Watson statistic of the AR(2) mol is greater th1.75. B is correct. There are three requirements for a time series to covarianstationary. First, the expectevalue of the time series must constant anfinite in all perio. Secon the varianof the time series must constant anfinite in all perio. Thir the covarianof the time series with itself for a fixenumber of perio in the past or future must constant anfinite in all perio. Martinez conclus ththe meanvarianof the time series of WTI oil prices are not constant over time. Therefore, the time series is not covarianstationary. 麻烦一下C,不太懂为啥不选它

2023-10-05 12:52 1 · 回答

4. Martinez’s Conclusion 1 is: correct. incorrebecause the meanvarianof WTI oil prices are not constant over time. incorrebecause the rbin–Watson statistic of the AR(2) mol is greater th1.75. B is correct. There are three requirements for a time series to covarianstationary. First, the expectevalue of the time series must constant anfinite in all perio. Secon the varianof the time series must constant anfinite in all perio. Thir the covarianof the time series with itself for a fixenumber of perio in the past or future must constant anfinite in all perio. Martinez conclus ththe meanvarianof the time series of WTI oil prices are not constant over time. Therefore, the time series is not covarianstationary. 这题从Exhibit 2看出是有AR现象,是不是有没有AR现象和covariance-stationary 3个条件是否constant & finite没有关系? 但如果有ARCH现象和单位根,那么则不是 covariance-stationary。还是只有单位根会违背 covariance-stationary ? 这里学的有点混乱,谢谢

2020-08-27 10:44 2 · 回答

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2019-12-20 03:44 1 · 回答

4. Martinez’s Conclusion 1 is: correct. incorrebecause the meanvarianof WTI oil prices are not constant over time. incorrebecause the rbin–Watson statistic of the AR(2) mol is greater th1.75. B is correct. There are three requirements for a time series to covarianstationary. First, the expectevalue of the time series must constant anfinite in all perio. Secon the varianof the time series must constant anfinite in all perio. Thir the covarianof the time series with itself for a fixenumber of perio in the past or future must constant anfinite in all perio. Martinez conclus ththe meanvarianof the time series of WTI oil prices are not constant over time. Therefore, the time series is not covarianstationary. the meanvarianof the time series of WTI oil prices are not constant over time. ---这句话从哪个指标可以看出来?

2019-11-08 02:49 1 · 回答