centre a table in latex with headings above cells

Question

I have a table I wish to fit in the margins. I also wish to add both EPS and PPS above the middle of the cells with these two in. I am not sure why adding a caption is not working either. Here is the code:

  \begin{tabular}{ |p{3cm}||p{3cm}|p{3cm}|p{3cm}|p{3cm}|  }
   \multicolumn{5}{c}{} \\
 \hline
&EPS:Pre &EPS:Post &PPS:Pre & PPS:Post \\
\hline
 Species tested:&13 &15& 43& 43\\
 Compounds tested:& 745 & 745& 310& 361 \\
Unique tests:& 193& 193& 406& 407\\
  Total experiments:&17,811 &17,929& 107,470& 130,926\\ 
   \hline
   \end{tabular}
   \label{tab}

Thank you.

Answer 1

There where several problems.

1. Your table was too large, with the margins and intercolumn spacing. I made a macro to fine adjust column width.

2. To redefine a column (for instance to center a header), you can use `\multicolumn{1}{c}{header}

3. \label assigns a number that is used in a numbered environment. It is associated with the table environmenent, not with tabular. The label appears within a caption. Also, the table is centered within the table environment.

\documentclass{article}

\begin{document}
\newcommand{\colwidth}{0.17\textwidth}
\newcommand{\centercolumn}[1]{\multicolumn{1}{c|}{#1}}
\begin{table}
  \centering
  \begin{tabular}{ |p{3cm}||p{\colwidth}|p{\colwidth}|p{\colwidth}|p{\colwidth}|  }
    % \multicolumn{5}{c}{} \\ useless, I think
    \hline
    &\centercolumn{EPS:Pre} &\centercolumn{EPS:Post} &\centercolumn{PPS:Pre} & \centercolumn{PPS:Post} \\
    \hline
    Species tested:&13 &15& 43& 43\\
    Compounds tested:& 745 & 745& 310& 361 \\
    Unique tests:& 193& 193& 406& 407\\
    Total experiments:&17,811 &17,929& 107,470& 130,926\\ 
    \hline
  \end{tabular}
  \caption{My table}
  \label{tab:1}
\end{table}
\end{document}