confusion matrix with [ ] delimeters

Question

I would like to make a confusion matrix that looks like the one on the picture. How can I achive this? Thanks

Answer 1

Quite hard coded, but you may start with this:

% arara: pdflatex

\documentclass{article}
\usepackage{blkarray}
\usepackage{multirow}
\usepackage{graphicx}
\usepackage{mathtools}

\begin{document}    
\[
\renewcommand{\arraystretch}{1.6}
\begin{blockarray}{c@{}ccc}
&&\BAmulticolumn{2}{c}{\text{observed}}\\[-2\jot]
&& 0 & 1 \\
\begin{block}{c@{\hspace{.6em}}c[cc]}
\multirow{2}{*}{\rotatebox[origin=c]{90}{predicted}} & 0 & a=1 & b=2\\
& 1 & c=3 & d=4 \\
\end{block}
\end{blockarray}
\]
\end{document}

Answer 2

With {NiceMatrix} of the eponymous package.

\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\renewcommand{\arraystretch}{1.2}
$\begin{NiceMatrix}
  &   & \Block{1-2}{\text{observed}} \
  &   & 0   & 1 \
  \Block{2-1}{\rotate \text{predicted}}
  & 0 & a=1 & b=2 \[1mm]
  & 1 & c=2 & d=4 
\CodeAfter 
  \SubMatrix[{3-3}{4-4}]
\end{NiceMatrix}$
\end{document}