Converting all numbers in document to set number of decimal places

See page 57 of the siunitx documentation.

A small example:

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\sisetup{
  table-number-alignment = center,
  table-figures-integer = 1,
  table-figures-decimal = 2
}
\begin{tabular}{
    S[table-auto-round]
  }
  2.\\
  2.3\\
  2.34\\
  2.345
\end{tabular}
\end{document}

You have to define each column with decimal figures as an S-columns.

You have 31 columns in your tabular, some with decimal, some with a mathematical sign and some empty. In the empty one, I have put a emdash (to keep track).

Here is a MWE (minimal working example). I used scrartcl with a high DIV-factor and landscape mode to squeeze the tabular in between the margin of a page. You can achieve the same by using the geometry-package with correct parametre:

\documentclass[a4paper,landscape,DIV=20]{scrartcl}
\usepackage{siunitx}
\begin{document}
\sisetup{
  table-number-alignment = center,
  table-figures-integer = 1,
  table-figures-decimal = 2
}
\begin{table}[thb]
\caption{\label{labelname}Table Caption}
\centering\tiny
\begin{tabular}{
    l
    S[table-auto-round]
    c
    S[table-auto-round]
    S[table-auto-round]
    c
    S[table-auto-round]
    l
    S[table-auto-round]
    c
    S[table-auto-round]
    l
    S[table-auto-round]
    c
    S[table-auto-round]
    l
    S[table-auto-round]
    c
    S[table-auto-round]
    l
    S[table-auto-round]
    c
    S[table-auto-round]
    c
    S[table-auto-round]
    c
    S[table-auto-round]
    c
    S[table-auto-round]
    c
    S[table-auto-round]
    l}

Dataset & \multicolumn{3}{c}{ConjunctiveRule }& \multicolumn{4}{c}{DecisionTable } &
\multicolumn{4}{c}{DTNB } & \multicolumn{4}{c}{JRip} & \multicolumn{4}{c}{NNge} &
\multicolumn{4}{c}{OneR} & \multicolumn{4}{c}{PART} & \multicolumn{4}{c}{Ridor} \\

Number correct & 
9.5900 & 
$\pm$ & 
1.16 & 
9.5600 & 
$\pm$ &
1.22 &
--- &
9.5600 &
$\pm$ &
1.22 &
--- & 
9.5000 & 
$\pm$ & 
1.24 &
--- &
8.9500 & 
$\pm$ &
 1.34 &
--- & 
9.6000 & 
$\pm$ & 
1.16 & 
--- & 
9.3000 & 
$\pm$ 
& 1.34 & 
--- & 
9.1700 & 
$\pm$ &
 1.42 &
--- \\ 
\end{tabular}
\end{table}
\end{document}