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\begin{document}
  
  
  \begin{tabular}{llS[table-format = 1.3]@{\extracolsep{1cm}}S[table-format = 1.3,retain-explicit-plus]}
    \toprule
    \multicolumn{2}{c}{Source of uncertainty} & \multicolumn{1}{c}{$f_ D\xspace\xspace^0\xspace$} & \multicolumn{1}{c}{$f_ D^{**0}\xspace$} \\
    \midrule
    \multirow{2}{*}{Statistical} & \text{OS} $ K\xspace$ \xspace sample & 0.025  & 0.027 \\
    & Templates & 0.047 & 0.052 \\
    \midrule
    \multirow{3}{*}{Floating syst.} & Signal form-factors & 0.006 & 0.004 \\
    & Non- $ B\xspace\xspace^-$ \xspace,  $\kern 0.18em\overline{\kern -0.18em  B\xspace}{}\xspace{}^0$ \xspace backgrounds & 0.004 & 0.004 \\
    &  $ B\xspace\xspace^-$ \xspace,  $\kern 0.18em\overline{\kern -0.18em  B\xspace}{}\xspace{}^0$ \xspace background normalization & 0.003  & 0.015 \\
    &  $\kern 0.18em\overline{\kern -0.18em  B\xspace}{}\xspace{}^0$ \xspace fraction and  $ m_{\mathrm{miss}}^2$ \xspace shape & 0.004 & 0.030 \\
    \midrule
    \multirow{2}{*}{Fixed syst.} &  $ D^{**0}$ \xspace branching fractions & 0.025 & 0.044 \\
    & Relative signal efficiency & 0.003 & 0.003 \\
    \midrule 
    \multicolumn{2}{l}{\multirow{2}{*}{Total uncertainty }} & {\multirow{2}{*}{0.056}} & +0.070 \\
    & & & -0.074 \\
    \bottomrule
  \end{tabular}
  
\end{document}
