Latex table multicolumn values are not centerd - latex

Question

I'm trying to create a table with the following layout:
This is what I was able to do so far, as you can see there is too much space in my multicolumns(circled in red)
My code is:
\begin{table*}
\caption{Different sampling strategies and the corresponding PRF scores,
explain the scores!}
\label{table:PRF_scores}
\centering
\begin{tabular}{c c | c c | l l l l } % 7 columns
\hline\hline
\multicolumn{2}{c}{\emph{sampling strategy}} & \multicolumn{2}{c}{} &
\multicolumn{4}{c}{\emph{performance metrics}} \\
under\_sample & over\_sample & N_{memb} & N_{non-memb} & F1(\%) &
ROC\_AUC(\%) PR\_AUC(\%) & MCC(\%)\\
\hline
0.5 & 1.0 & 752 & 752 & 98.54 \pm 0.62 & 98.62 \pm 0.64 & 98.62 \pm 0.59
& 97.08 \pm 1.25\\
0.1 & 1.0 & 3760 & 3760 & 99.09 \pm 0.17 & 99.08 \pm 0.18 & 99.09 \pm
0.17 & 98.17 \pm 0.35\\
0.06 & 0.6 & 3759 & 6266 & 98.68 \pm 0.23 & 99.19 \pm 0.15 & 98.70 \pm
0.23 & 97.89 \pm 0.37\\
\hline
\end{tabular}
\end{table*}
What can I do to fix this issue?
Any help is greatly appreciated!

Answer 1

I propose a "re-designed" table.
First, I would avoid vertical bars, which IMO are rather distraction. Then, I moved group of N to the left; it doesn't seem to be a part of the other two groups and shouldn't be in the middle, does it? I also reduced wording in a few columns and added line breaks in long headers. This is one way to avoid overly wide and unbalanced tables.
Here is a list of helpful packages I applied to help me to format your table:
array define column groups and additional column types
makecell format cells and allows for line breaks, which I haven't had to use here
threeparttable splits a table into three parts: caption, body, and annotation; also, keeps a table and its caption within boundaries
caption adds options to format a caption
siunitx formats and align numbers
booktabs adds improved horizontal rules
Here's the screenshot
and the code
\documentclass{article}
\usepackage{array}
\usepackage{threeparttable}
\usepackage{booktabs}
\usepackage{makecell}
\usepackage{caption}
\usepackage{siunitx}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}
\begin{document}
\begin{table}[tbh]
\small
% Settings for the caption
\captionsetup{position=top, skip=3pt, format=hang}
% Reduced horizontal spaces between columns
\setlength\tabcolsep{3pt}
% Slightly increased vertical spacing
\renewcommand{\arraystretch}{1.2}
% Settings for makecell
\renewcommand{\theadfont}{\small\bfseries\itshape}
\renewcommand{\theadgape}{\Gape[0pt][0pt]}
\renewcommand{\cellgape}{\Gape[3pt][0pt]}
%
\centering
\begin{threeparttable}
\caption{Different sampling strategies and the corresponding PRF scores, explain the scores!}
\label{table:PRF_scores}
\begin{tabular}{
*2{S[table-format=4]}
#{\quad}
*2{S[table-format=1.2]}
*4{C{1.8cm}}
}
\toprule
\multicolumn{2}{c}{} & \multicolumn{2}{c}{\thead{Strategy}} & \multicolumn{4}{c}{\thead{Performance metrics}} \\
\cmidrule{3-4}\cmidrule(l){5-8}
\multicolumn{2}{c}{N$^{*}$} & \multicolumn{2}{c}{Sample} & F1 & ROC & PR & MCC \\[-3pt]
{mem} & {n-mem} & {under} & {over} & (\%) & AUC (\%) & AUC (\%) & (\%) \\
\midrule
752 & 752 & 0.5 & 1.0 & 98.54 \pm 0.62 & 98.62 \pm 0.64 & 98.62 \pm 0.59 & 97.08 \pm 1.25 \\
3760 & 3760 & 0.1 & 1.0 & 99.09 \pm 0.17 & 99.08 \pm 0.18 & 99.09 \pm 0.17 & 98.17 \pm 0.35\\
3759 & 6266 & 0.06 & 0.6 & 98.68 \pm 0.23 & 99.19 \pm 0.15 & 98.70 \pm 0.23 & 97.89 \pm 0.37\\
\bottomrule
\multicolumn{8}{#{}l#{}}{%
\makecell[l]{\footnotesize$^{*}$ The left column represents members of a group; the right
column the opposite}}
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}