To complete the interval, one needs a specific value for
\(z_{ \alpha/2}\) using an
inverse normal distribution calculator [STRUCT].[NUM]. Generally, one chooses confidence levels on the order of 90%, 95%, or 99% with 95% being the usual choice. Fortunately this value is easily computed using graphing calculators or other automatic methods although your ancient teacher might have been required to use tables. On a TI calculator, use
\begin{equation*}
z_{\alpha /2} = \text{InvNorm}( 1 - \frac{\alpha}{2} )
\end{equation*}