How to align numbers according to \pm but not according to the points?

Question

How to align numbers according to \pm but not according to the points? Each row should also have a different number of decimal places.

\documentclass{article}
\usepackage{siunitx}


\begin{document}
\begin{table}
\caption{}
\centering
\small
\sisetup{separate-uncertainty}
\begin{tabular}{lS[table-format = 5.4(1)]}
\hline\hline\noalign{\smallskip}
Parameter & Value \\
\noalign{\smallskip}\hline\noalign{\smallskip} 
A   & 26.5 \pm 2.0      \\
B   & 11.102 \pm 1.2 \\
C   & 53839  \pm 550 \\
D   & 0.5863     \pm 0.016 \\

\noalign{\smallskip}\hline\noalign{\smallskip}
\end{tabular}
\end{table}

\end{document}

Answer 1

As this answer suggests, the S column provided by the package siunitx only comes with certain alignment functions that primarily adjust numbers to the decimal separator, which is not what you want.

I therefore would suggest that you adjust the nice answer linked above and do the alignment manually without the use of the siunitx package. Since I don't know how you wish to align the numbers behind the \pm sign, I provide below two alternative solutions. I would also suggest to have a look at the booktabs package:

\documentclass{article}
%\usepackage{siunitx}
\usepackage{booktabs}

\begin{document}

\begin{tabular}{ l r @{\,\( \pm \)\,} r }
\toprule
{Parameter} & \multicolumn{2}{c}{Value} \\
\midrule
A   & 26.5   & 2.0   \\
B   & 11.102 & 1.2   \\
C   & 53839  & 550   \\
D   & 0.5863 & 0.016 \\
\bottomrule
\end{tabular}

\bigskip

\begin{tabular}{ l r @{\,\( \pm \)\,} l }
\toprule
{Parameter} & \multicolumn{2}{c}{Value} \\
\midrule
A   & 26.5   & 2.0   \\
B   & 11.102 & 1.2   \\
C   & 53839  & 550   \\
D   & 0.5863 & 0.016 \\
\bottomrule
\end{tabular}

\end{document}

Answer 2

I am not aware of siunitx having implemented any feature to centre numbers around uncertainty markers. Perhaps it has and I don't know about it. The only workaround I can think of is to split numbers in two columns to get the effect. You could still use siunitx to let's say round numbers to some decimal places. Other than that, siunitx is not required

\documentclass{article}
\usepackage{siunitx}
\newcommand\ustrut{\rule{0pt}{12pt}}
\newcommand\lstrut{\rule[-6pt]{0pt}{12pt}}


\begin{document}
\begin{table}
  \sisetup{
    round-mode=figures,
    round-precision=3,
    table-alignment-mode=none,
    separate-uncertainty,
  }
  \caption{}
  \centering
  \small
  \begin{tabular}{
      l
      S[table-number-alignment=right]
      @{\(\;\pm\;\)}
      S[table-number-alignment=left]
    }
    \hline\hline
    \ustrut Parameter & \multicolumn{2}{c}{Value\lstrut}\\
    \hline\ustrut
    A   &   26.5 & 2.0 \\
    B   & 11.102 & 1.2 \\
    C   &  53839 & 550 \\
    D   & 0.5863 & 0.016 \lstrut \\
    \hline
  \end{tabular}
  \end{table}
\end{document}