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Yan · 2021年03月21日

这里的autocorrelation是指什么

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NO.PZ201709270100000506

问题如下:

6. Based on the regression output in Exhibit 3 and sales data in Exhibit 4, the forecasted value of quarterly sales for March 2016 for PoweredUP is closest to:

选项:

A.

$4.193 billion.

B.

$4.205 billion.

C.

$4.231 billion.

解释:

C is correct. The quarterly sales for March 2016 is calculated as follows:

beginarraylln Salest-ln Salest-1=b0+b1(ln Salest-1-ln Salest-2)+b2(ln Salest4-ln Salest-5)ln Salestln3.868=0.00920.1279(ln3.868ln3.780)+0.7239(ln3.836ln3.418)ln Salest=1.44251Salest=e1.44251=4.231begin{array}{l}\ln{\text{ Sales}}_\text{t}{\text{-ln Sales}}_\text{t-1}{\text{=b}}_\text{0}{\text{+b}}_\text{1}{\text{(ln Sales}}_\text{t-1}{\text{-ln Sales}}_\text{t-2}{\text{)+b}}_\text{2}\text{(ln Sales}_{t-4}^{}{\text{-ln Sales}}_\text{t-5}\text{)}\\\ln{\text{ Sales}}_\text{t}-\ln3.868=0.0092-0.1279(\ln3.868-\ln3.780)+0.7239(\ln3.836-\ln3.418)\\\ln{\text{ Sales}}_\text{t}=1.44251\\{\text{Sales}}_\text{t}=e^{1.44251}=4.231

这里EXIBIT3的AUTOCORRELATION指的是什么?能具体说一下么?怎么这LAG4是不显著的,而上面EXIBIT2中的LAG4是显著的?

1 个答案

星星_品职助教 · 2021年03月21日

同学你好,

autocorrelation指的是残差项之间的相关性。具体而言,是滞后的残差项εt-1(一直到εt-4)和εt之间彼此的相关性。

Exhibit 3中,四项残差项对应的t-statistic的绝对值都小于t critical value的绝对值(根据Exhibit 3的notes可知t critical value=2.03),所以这四项都不能拒绝ρ=0的原假设,即这四项和εt的相关性都是0。也就是no autocorrelation。

而Exhibit 2中,第四项εt-4和εt的t-statistic的绝对值等于4.3111,大于了此时的t critical value2.02,所以是拒绝ρ=0的原假设,即ρ≠0。这就有了相关性,所以违背了no autocorrelation的假设。

Exhibit 2和3结果不同的原因是Exhibit 3对应的方程进行了修正,加入了一项,反应了出了季节性因素,所以再检测autocorrelation就没问题了。

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