\begin{table}[hb]
 \small
 \centering
  \begin{tabular}{c|c|c|c|c|c|c|c|c}
  \toprule
 \makebox [0.083\textwidth][c]{}  &  \makebox[0.083\textwidth][c]{Day1}  & \makebox[0.083\textwidth][c]{ Day2}      & \makebox[0.083\textwidth][c]{ Day3} & \makebox[0.083\textwidth][c]{ Day4}  & \makebox[0.083\textwidth][c]{ Day5}  & \makebox[0.083\textwidth][c]{ Day6}  & \makebox[0.083\textwidth][c]{ Day7} & \makebox[0.083\textwidth][c]{ Average}\\
\midrule
    $\triangle$ CTR    & 1.43\%    & 1.56\%  & 1.79\%  & 1.82\% & 1.78\% & 1.58\% & 1.37\% & 1.62\% \\
\bottomrule
  \end{tabular}
   \caption{Results of a seven-day online A/B test.}
 \label{abtest_7}
\end{table}