尝试:
\documentclass[a4paper,12pt]{article}
\usepackage{amsmath}
\usepackage{booktabs}
\begin{document}
\begin{tabular}{cc}
\toprule
& Variable $1$ \\\midrule
Item 1 & $x_{11}$\\
Item 2 & $x_{21}$\\
$\vdots$ & $\vdots$\\
Item $j$ & $x_{j1}$\\
\bottomrule
\end{tabular}
\end{document}