How to format this long table?

Question

With the current version of my codes, the table looks a bit off.

    \documentclass{article}  
    \usepackage{longtable}
    
    \title{A longtable example}
    
    \begin{document}
    \begin{center}
    \begin{longtable}[H]{|p{2.5cm}|p{1.2cm}|p{1.2cm}|p{1.2cm}|p{1.2cm}|p{1.2cm}|p{1.2cm}|}
        \caption{WITS and ASUS Type of Treatment by Offender Type}
        \label{test_table} \\ \hline
        &\multicolumn{3}{c|}{\textbf{WITS}}&\multicolumn{3}{c|}{\textbf{ASUS}}\\ \hline
        \textbf{Treatment \textbackslash{} Offender Type} & \textbf{Probationer} & \textbf{Parolees} &\textbf{Maximum-term Released Prisoners}  & \textbf{Probationer} & \textbf{Parolees} &\textbf{Maximum-term Released Prisoners} \\ \hline
        Outpatient&  &  &  & &  & \\ \hline
        Intensive Outpatient&  &  &  &  &  &\\ \hline
        Day Treatment&  &  &  & &  & \\ \hline
        Partial Hospitalization&  &  &  &  &  &\\ \hline
        Short Term Residential&  &  &  &  &  &\\ \hline
        Inpatient&  &  &  &  &  &\\ \hline
        Long Term Residential&  &  &  &  &  &\\ \hline
        Therapeutic Community&  &  &  &  &  &\\ \hline
        \end{longtable}
    \end{center}
    \end{document}

See here for the output:

Any suggestions how I can format it and make it more presentable? I have been trying adjusting the widths of the table but to no avail.

EDIT: I tried something like this:

> %\documentclass{article}
> %\usepackage{longtable}
> 
> %\title{A longtable example}
> 
> %\begin{document}
> 
> %\begin{center}
> \begin{longtable}[H]{|p{2.5cm}|p{1.2cm}|p{1.2cm}|p{1.5cm}|p{1.2cm}|p{1.2cm}|p{1.5cm}|}
>   \caption{WITS and ASUS Type of Treatment by Offender Type}
>   \label{ASUS-WITS_treatment_table2}  \\ \hline
>   &\multicolumn{3}{c|}{\textbf{WITS}}&\multicolumn{3}{c|}{\textbf{ASUS}}\\
> \hline    \textbf{Treatment \textbackslash{} Offender Type} &
> \textbf{Prob.} & \textbf{Parol.} &\textbf{Max-term Rel. Pris.}  &
> \textbf{Prob.} & \textbf{Parol.} &\textbf{Max-term Rel. Pris.} \\
> \hline    Outpatient&  &  &  & &  & \\ \hline     Intensive Outpatient&  & 
> &  &  &  &\\ \hline   Day Treatment&  &  &  & &  & \\ \hline  Partial
> Hospitalization&  &  &  &  &  &\\ \hline  Short Term Residential&  & 
> &  &  &  &\\ \hline   Inpatient&  &  &  &  &  &\\ \hline  Long Term
> Residential&  &  &  &  &  &\\ \hline  Therapeutic Community&  &  &  & 
> &  &\\ \hline     \multicolumn{7}{@{}p{\textwidth}@{}}{Prob. =
> Probationer; Parol. = Parolees; Max.-term Rel. Pris. = Maximum-term
> Released Prisoners}\\     \end{longtable} %\end{center}
> 
> %\end{document}

Answer 1

You had specified the table to be wider than the page, so I reduced the column widths.

It would look better with shorter top headings, but you need subject knowledge to pick better headings so I left them as they are.

\documentclass{article}

\usepackage{longtable,array}

\title{A longtable example}

\begin{document}
\setlength\extrarowheight{2pt}
\begin{longtable}{|>{\raggedright}p{2.3cm}|p{1.1cm}|p{1.1cm}|p{1.1cm}|p{1.1cm}|p{1.1cm}|p{1.1cm}|}
    \caption{WITS and ASUS Type of Treatment by Offender Type}
    \label{test_table} \\ \hline
    &\multicolumn{3}{c|}{\textbf{WITS}}&\multicolumn{3}{c|}{\textbf{ASUS}}\\ \hline
    \textbf{Treatment \textbackslash{} Offender Type} & \textbf{Prob\-ationer} & \textbf{Parol\-ees} &\textbf{Maxi\-mum term Released Prisoners}  & \textbf{Prob\-ationer} & \textbf{Parol\-ees} &\textbf{Maxi\-mum term Released Prisoners} \\ \hline
    Outpatient\\\mbox {}&  &  &  & &  & \\ \hline
    Intensive Outpatient&  &  &  &  &  &\\ \hline
    Day Treatment&  &  &  & &  & \\ \hline
    Partial Hospitalization&  &  &  &  &  &\\ \hline
    Short Term Residential&  &  &  &  &  &\\ \hline
    Inpatient\\\mbox{ }&  &  &  &  &  &\\ \hline
    Long Term Residential&  &  &  &  &  &\\ \hline
    Therapeutic Community&  &  &  &  &  &\\ \hline
    \end{longtable}

\noindent X\dotfill X

\end{document}

Answer 2

I propose this, which uses the xltabular environment, which combines the functionalities of longtable and tabularx.

Also, depending on the real contents of your table, you might consider having a landscape table.

\documentclass[11pt, letterpaper]{article}
\usepackage[margin=0.75in]{geometry}
\usepackage{caption}
\usepackage{longtable}
\usepackage{xltabular}
\renewcommand{\tabularxcolumn}[1]{m{#1}}
\usepackage{ragged2e}
\newcolumntype{Y}{>{\hsize=0.7\hsize\RaggedRight\arraybackslash}X}
\title{A longtable example}

\begin{document}

\setlength{\extrarowheight}{2pt}\setlength{\tabcolsep}{2pt}
\captionsetup{width=\linewidth}
\begin{xltabular}{\linewidth}{|>{\hsize=1.6\hsize\RaggedRight}X|*{2}{X|Y|X|}}
    \caption{WITS and ASUS Type of Treatment by Offender Type}
    \label{test_table} \\ \hline
    &\multicolumn{3}{c|}{\textbf{WITS}}&\multicolumn{2}{c|}{\textbf{ASUS}}\\ \hline
    \textbf{Treatment \textbackslash{}\newline Offender Type} & \textbf{Probationer} & \textbf{Parolees} &\textbf{Maximum-term Released Prisoners} & \textbf{Probationer} & \textbf{Parolees} &\textbf{Maximum-term Released Prisoners} \\ \hline
    Outpatient& & & & & & \\ \hline
    Intensive Outpatient& & & & & &\\ \hline
    Day Treatment& & & & & & \\ \hline
    Partial Hospitalization& & & & & &\\ \hline
    Short Term Residential& & & & & &\\ \hline
    Inpatient& & & & & &\\ \hline
    Long Term Residential& & & & & &\\ \hline
    Therapeutic Community& & & & & &\\ \hline
    \end{xltabular}

\end{document}