Table contents are not centring properly

Question

I'm trying to make a table however the contents of certain columns aren't centred correctly.

I'm using the following in my preamble...

\documentclass[11pt,a4paper,twoside, twocolumn]{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[UKenglish]{babel}
\usepackage[UKenglish]{isodate}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{lmodern}
\usepackage{bm}
\usepackage{url}
\usepackage{paralist}
\usepackage{lipsum}
\usepackage{siunitx}
\usepackage{lettrine}
\usepackage{graphicx}

\usepackage{booktabs}
\usepackage{multirow}
\usepackage{array}
\newcolumntype{L}{>{\centering\arraybackslash}m{2.75cm}}

\usepackage{todonotes}

\usepackage{titling}
\setlength{\droptitle}{-2em} 

\title{}
\author{}
\date{}

\usepackage{csquotes}

\usepackage[style=authoryear, backend=biber]{biblatex}
\addbibresource{phage.bib}

\usepackage{cleveref}
\usepackage[font=footnotesize,labelfont={bf,sf},justification=centering]{caption}
\usepackage{fancyhdr} 
\usepackage{microtype}

And the code so far for my table is as follows:

\begin{table*}[t]
\centering
\begin{tabular}{ccc}
\toprule
Treatment & \multicolumn{2}{c}{No. of survivors/total no. of mice (\% survival)} \\
\midrule
& \multicolumn{2}{c}{Combined experiment} \\
& 48 hpi & 72 hpi \\
\midrule
PAO1 only & Data here & Data here\\
\bottomrule
\end{tabular}
\label{micesurvival}
\caption{Caption here}
\end{table*}

Which outputs like this:

As you can see the 48hpi and 72hpi columns and the following data below aren't centred properly beneath the combined experiment multicolumn.

It's probably something obvious I've overlooked..

Answer 1

Besides using the tabular* environment, you could also use the tabularx environment -- provided by the package with the same name. The X column type provided by the package is quite useful for your purpose, as the following modified form of your MWE shows:

\documentclass{article}
\usepackage{booktabs}
\usepackage{tabularx}
\newcolumntype{C}{>{\centering\arraybackslash}X} % centered "X" column type
\begin{document}
\begin{table}
\centering
\begin{tabularx}{0.82\textwidth}{cCC} % 0.82 obtained empirically
\toprule
Treatment & \multicolumn{2}{c}{No. of survivors/total no. of mice (\% survival)} \\
\midrule
& \multicolumn{2}{c}{Combined experiment} \\
\cmidrule{2-3} % additional element
& 48 hpi & 72 hpi \\
\midrule
PAO1 only & Data here & Data here\\
\bottomrule
\end{tabularx}
\caption{Caption here}\label{micesurvival}
\end{table}
\end{document}

Note also that it's important to place the \label command after the \caption command. Incidentally, I've also changed the floating environment from table* to table; using a "starred" table or figure environment is only useful if your document has two (or more) columns per page and you need to have a float that spans all columns.

Answer 2

It's a feature of the underlying \halign primitive that if a spanning cell is wider than the cells it spans then all the extra width goes into the last spanned column.

One way to avoid that is to ensure that the natural width of the columns is very wide but shrinkable by adding shrinkable glue:

A second way is to force each of the columns to be half the width of the wide heading as shown in the second table.

\documentclass{article}
\usepackage{booktabs,array}
\begin{document}


\centering

\begin{tabular*}{\linewidth}{
@{\extracolsep{\linewidth minus \linewidth}}
ccc
@{}
}
\toprule
Treatment & \multicolumn{2}{@{}c@{}}{No. of survivors/total no. of mice (\% survival)} \\
\midrule
& \multicolumn{2}{@{}c@{}}{Combined experiment} \\
& 48 hpi & 72 hpi \\
\midrule
PAO1 only & Data here & Data here\\
\bottomrule
\end{tabular*}

\bigskip
\newlength\myl
\settowidth\myl{No. of survivors/total no. of mice (\% survival)}

\begin{tabular}{
c
>{\centering\arraybackslash}p{.5\myl}
>{\centering\arraybackslash}p{.5\myl}
}
\toprule
Treatment & \multicolumn{2}{c}{No. of survivors/total no. of mice (\% survival)} \\
\midrule
& \multicolumn{2}{c}{Combined experiment} \\
& 48 hpi & 72 hpi \\
\midrule
PAO1 only & Data here & Data here\\
\bottomrule
\end{tabular}




\end{document}

Please always post complete documents showing all packages used.