\documentclass[12pt]{article}% Herbert Voss 20030813
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{nicefrac}
\usepackage{dcolumn}
\newcolumntype{.}{D{.}{.}{-1}}
\newcommand\ds{\displaystyle}
\begin{document}

{\def\arraystretch{2}
\[
\begin{array}{c|c..}
     & r_{t+1}^a & \dfrac{v^a_{t+1}}{v^a_t} & \dfrac{s^a_t}{v^a_t}\\\hline
  r_{t+1}^a & 1 & -0.8388 & -0.0824 \\
  \dfrac{v^a_{t+1}}{v^a_t} & & 1 & 0.6144 \\
  \dfrac{s^a_t}{v^a_t} & & & 1 \\
\end{array}
\]
}
{\def\arraystretch{1.4}
\[
\begin{array}{c|c..}
 & r_{t+1}^a & \nicefrac{\ds v^a_{t+1}}{v^a_t}
   & \nicefrac{\ds s^a_t}{v^a_t}\\\hline
   r_{t+1}^a & 1 & -0.8388 & -0.0824 \\
   \nicefrac{\ds v^a_{t+1}}{\ds v^a_t} & & 1 & 0.6144 \\
   \nicefrac{\ds s^a_t}{\ds v^a_t} & & & 1 \\
\end{array}
\]
}

\end{document}