上段落中提到 95.4% 的涵蓋區域, 我們用 $100(1-\alpha)\%$ 表示, 以後均以 $1-\alpha$ 來表示 .95 或是 .90 等等.

$z_{\alpha/2}=$ the upper $\alpha/2$ point of the standard normal distribution. That is, the area to the right of $z_{\alpha/2}$, and the area between $-z_{\alpha/2}$ and $z_{\alpha/2}$ is $1-\alpha$, as follow.

下面列出 $z_{\alpha/2}$ 幾個常見的值,

$\begin{array}{c\vert ccccc} \hline 1-\alpha&.80&.85&.90&.95&.99 \\ \hline z_{\alpha/2}&1.28&1.44&1.645&1.96&2.58 \\ \hline \end{array}$