difference这里，-5到-8的difference是3，-8到1的difference是-9，这是为啥？什么时候用绝对值？

问题如下：

Question

An analyst collects the following data related to paired observations for Sample A and Sample B. Assume that both samples are drawn from normally distributed populations and that the population variances are not known.

The t-statistic to test the hypothesis that the mean difference is equal to zero is closest to:

选项：

A.0.52. B.0.27. C.0.23.

解释：

Solution

A is correct.

$t=\frac{\bar{d}?{\mu }_{d0}}{s_{\bar{d}}}$

In this case, the mean difference is 1.4. The sample variance is 36.8. The standard error of the mean difference is 2.712932. The t-statistic is 0.51605 ~ 0.52.

7/5 = mean difference (ˉd)$\left(\bar{d}\right)$ of 1.4

Standard Error (sˉd)$\left(s_{\bar{d}}\right)$: (36.8/5)^0.5 = 2.712932

t-statistic: (1.4 - 0)/2.712932 = 0.51605 ~ 0.52