Centering large column heading above 3 columns

Question

After searching for hours I could not solve my problem. I have a large table with means and sd separated in three columns. I need to three below columns to be centered on the +-.

I tried using S tables but when I use S type tables I receive an error message with the column header on two lines. Any other simple solution keeping the 3 column form of the mean +- sd.

Here is a short extract of the table.

Thank you for your help.

\begin{table}[]
\begin{tabular}{|l|r|c|l|}
\textit{Acides gras (\%)} & \multicolumn{3}{c}{\begin{tabular}[c]{@{}c@{}}My long three column and two lines header to be centered \\ (n = 91)\end{tabular}} \\
\textit{Saturés}          &                               &                             &                               \\
14:0                      & 2.6                           & ±                           & 0.7                          
\end{tabular}
\end{table}

! Edit

I am also there thank you ! Now my problem is that I need some rows to be in bold and when I use the bfseries in tblr only the first column is actually bold.

    \begin{table}
\sisetup{mode=text,           % <--
         separate-uncertainty % <--
        }
    \centering
\begin{tblr}{colspec = { X[l] *{6}{X[c, si={table-format=2.1(2)}]} },  % < ---
             colsep = 1pt,
             hspan  = minimal,
             row{1} = {guard},
             row{3} = {font=\bfseries}
            }
    \toprule
Acides gras (\%) 
        &   \SetCell[c=3]{c}    {My long three column and two lines header to be centered\\  ($n = 91$)}
            &   &   &   \SetCell[c=3]{c} {A shorter title\\ ($n = 91$)}
                        &   &               \\
        \cmidrule[lr]{2-4}
        \cmidrule[l]{5-7}
Saturés & & & & & & \\
14:0 & 2.6(0.7) & 1.8(0.7) & 2.2(0.6) & 6.9(0.7) & 4.3(1.7) & 3(0.8) \\
\end{tblr}
    \end{table}   

Answer 1

• I suspect that your table contain values with uncertainty. This should be written by help of siunitx package and not how you try to do.
• At use of tabularray package is advisable to strict use its syntax (no \newline, to each multi column cells should follows all ampersands of spanned columns).
• MWE below try to reproduce part of table shown in the second image in question:

\documentclass{article}
\usepackage{tabularray}
\UseTblrLibrary{booktabs, siunitx}

\begin{document}
    \begin{table}
\sisetup{mode=text,           % <--
         separate-uncertainty % <--
        }
    \centering
\begin{tblr}{colspec = { X[l] *{6}{X[c, si={table-format=2.1(2)}]} },  % < ---
             colsep = 1pt,
             hspan  = minimal,
             row{1} = {guard}  % <--
            }
    \toprule
Acides gras (\%) 
        &   \SetCell[c=3]{c}    {My long three column and two lines header to be centered\\  ($n = 91$)}
            &   &   &   \SetCell[c=3]{c} {A shorter title\\ ($n = 91$)}
                        &   &               \\
        \cmidrule[lr]{2-4}
        \cmidrule[l]{5-7}
\textit{Saturés} 
        &   &   &   &   &   &               \\
14:0    & 2.6(0.7)  
            & 1.8(0.7)
                & 2.2(0.6)
                    & 6.9(0.7)
                        & 4.3(1.7)
                            & 3.0(0.8)      \\
16:0    & 6.6(1.6)
            & 5.4(2.0)
                & 6.9(1.7)
                    & 12.9(1.8)
                        & 12.9(5.6)
                            & 10.6(3.2)      \\
    \bottomrule
\end{tblr}
    \end{table}   
\end{document}

Answer 2

You could avoid the problem by using the tabularray package for your table:

\documentclass{article}

\usepackage{tabularray}

\begin{document}

\begin{table}[htbp]
\begin{tblr}{
  colspec={|X[l]|X[r]|c|X[l]|},
  vlines,
  cell{1}{1}={font=\itshape},
  hspan=minimal
}
Acides gras (\%) Saturés & \SetCell[c=3]{halign=c} My long three column and two lines header to be centered (n = 91) && \\
14:0 & 2.6 & ± & 0.7                          
\end{tblr}
\end{table}

\end{document}

Answer 3

This equalizes the three columns by precomputing the widths needed, assuming that you want the three columns the same width.

I also threw in my (aesthetic) version.

\documentclass{article}
\begin{document}

\begin{table}[htp]
\sbox0{\begin{tabular}[c]{@{}c@{}}My long three column and
  two lines header to be centered \\ (n = 91) \end{tabular}}% measure width
\setlength{\dimen0}{\dimexpr \wd0-4\tabcolsep}% compute column width
\divide\dimen0 by 3
\begin{tabular}{|l|r|c|l|}
\textit{Acides gras (\%)} & \multicolumn{3}{c|}{\usebox0} \\
\textit{Saturés}          & \hspace{\dimen0} & \hspace{\dimen0} & \hspace{\dimen0} \\
14:0                      & 2.6              & ±                & 0.7                          
\end{tabular}
\end{table}

\begin{table}[htp]
\sbox0{My long three column and two lines header to be centered}% measure width
\sbox1{ $\pm$ }% measure width of @ divider
\setlength{\dimen0}{\dimexpr 0.5\wd0-0.5\wd1}% compute column width
\begin{tabular}{|l|r@{\usebox1}l|}
\textit{Acides gras (\%)} & \multicolumn{2}{c|}{\usebox0} \\
\textit{Saturés}          & \multicolumn{2}{c|}{(n = 91)} \\
14:0                      & \makebox[\dimen0][r]{2.6} & \makebox[\dimen0][l]{0.7}% only need \makebox in one row
\end{tabular}
\end{table}

\end{document}