Multi-header column alignment with booktabs

You could give the table header more visual structure, along the lines of the following example, by creating an additional line. You're of course entirely free to modify the proposed overall header, "Actions regarding unlabeled detections".

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{booktabs}
\begin{document}
\begin{center}
\begin{tabular}{@{} l cccc @{}}
    \toprule
    & \multicolumn{4}{c@{}}{Actions regarding unlabeled detections}\\
    \cmidrule(l){2-5}
    & \multicolumn{2}{c}{Ignore} & \multicolumn{2}{c@{}}{Approve}\\
    \cmidrule(lr){2-3} \cmidrule(l){4-5}
    & Recall & Precision & Recall & Precision  \\
    \midrule
    count & 513     & 446         &  529     & 484     \\
    \addlinespace
    mean  & 0.428   & 0.787       &  0.507   & 0.841   \\
    std   & 0.343   & 0.365       &  0.337   & 0.310   \\
    \addlinespace
    min   & 0.000   & 0.000       &  0.000   & 0.000   \\
    $Q_1$ & 0.000   & 0.667       &  0.286   & 0.800   \\
    $Q_2$ & 0.500   & 1.000       &  0.500   & 1.000   \\
    $Q_3$ & 0.667   & 1.000       &  0.750   & 1.000   \\
    max   & 1.000   & 1.000       &  1.000   & 1.000   \\
    \bottomrule
\end{tabular}
\end{center}
\end{document} 

Here is a version that keeps the headers as they are and sets the column width of the second to fifth column with respect to the width of the corresponding multirow header.

However, I personally would prefer Mico's solution as there are quite large white spaces in my version of the table.

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{booktabs}
\usepackage{calc}
\usepackage{array}

\begin{document}

\begin{tabular}{@{}l*{4}{>{\centering\arraybackslash}p{\widthof{Approving Unlabeled Detections}/2 - 2\tabcolsep}}@{}}
    \toprule
        & \multicolumn{2}{@{}c}{Ignoring Unlabeled Detections} 
        & \multicolumn{2}{c@{}}{Approving Unlabeled Detections}\\
    \cmidrule(r){2-3} 
    \cmidrule(r){4-5}
        & Recall  & Precision    & Recall  & Precision  \\
    \cmidrule(r){2-3} 
    \cmidrule(r){4-5}
    % \midrule
    count           & 513     & 446         &  529     & 484     \\
    mean            & 0.428   & 0.787       &  0.507   & 0.841   \\
    std             & 0.343   & 0.365       &  0.337   & 0.310   \\
    min             & 0.000   & 0.000       &  0.000   & 0.000   \\
    $Q_1$           & 0.000   & 0.667       &  0.286   & 0.800   \\
    $Q_2$           & 0.500   & 1.000       &  0.500   & 1.000   \\
    $Q_3$           & 0.667   & 1.000       &  0.750   & 1.000   \\
    max             & 1.000   & 1.000       &  1.000   & 1.000   \\
    \bottomrule
\end{tabular}

\end{document}