[Tex/LaTex] left alignment of multicolumn inserts extra space

Question

how do I get \multicolumn to align left? I always get a small space at the beginning of the row. Here's a code

\documentclass[12pt]{article}

\usepackage{booktabs}
\usepackage{dcolumn}
\newcolumntype{d}[1]{D.{\mbox{.}}{#1}}      

\begin{document} 
\begin{table}       
\centering 
\begin{tabular}{@{} l d{2.3} d{2.3} @{}}                                       
\toprule \toprule                                                                                                                                                                                                                                                                                                                                     
\textbf{Benchmark} \\                                                                
$\quad $Net effect & 0.73 & 0.59  \\     
\multicolumn{3}{l}{\textbf{Benchmark}} \\                                                                
$\quad $Net effect & 0.73 & 0.59  \\     \bottomrule \bottomrule 
\end{tabular}                                                                                                                                                                                                                                     
\end{table}     

\end{document}

The second 'Benchmark' is not fully aligned to the left. How can I achieve that?

Answer 1

Note that the main tabular has the first column alignment specified as

@{} l

which removes the column spacing to the left of the left-aligned column (due to @{}). \multicolumn{.}{l} re-formats the column specification to l without the removal of the column spacing on the left. So you should either use

\begin{tabular}{ l ... }
  ...
  \multicolumn{3}{l}{..}
  ...
\end{tabular}

or

\begin{tabular}{ @{} l ... }
  ...
  \multicolumn{3}{@{} l}{..}
  ...
\end{tabular}