Here is a table but it has a problem: the column of 1, 20, 50, and 100 aren't perfectly aligned to others. How can I do that?
MWE
\documentclass{article}
\usepackage{siunitx,booktabs,multirow}
\begin{document}
\begin{table}[htp]
\centering
\begin{tabular}{lS[table-format=4.5]
S[table-format=4.5]
S[table-format=4.5]
S[table-format=4.5]}
\toprule
\multirow{2}*{\textsc{PEP}} & \multicolumn{4}{c}{Nodi} \\ \cmidrule(lr){2-5}
& 1 & 20 & 50 & 100 \\ \midrule
1e-06 & 1 & 1 & 1 & 1.0001 \\
1e-05 & 1 & 1.0002 & 1.0005 & 1.001 \\
0.0001 & 1.0001 & 1.002 & 1.005 & 1.0099 \\
0.001 & 1.001 & 1.0198 & 1.0486 & 1.0952 \\
0.01 & 1.01 & 1.1846 & 1.4 & 1.6447 \\
0.1 & 1.1111 & 2.084 & 2.4454 & 2.7404 \\ \bottomrule
\end{tabular}
\end{table}
\end{document}