here is my table but I can't figure out how to arrange 4 more column on the right side.
\begin{sidewaystable}
\centering
\caption{Realtime forecast RMSEs}
\label{forecast}
\begin{tabular}{@{}l
c
*{6}{S[table-format=1.6, group-digits=false]}
S[table-format=-1.6, group-digits=false]
S[table-format=1.6, group-digits=false]@{}}
\toprule
\multirow{2}[3]{*}{\bfseries Parameters} & \multicolumn{2}{c}{\makecell{\bfseries $S\&P$ \\ $\boldsymbol{h=2Q}$}} &
\multicolumn{2}{c}{\makecell{\bfseries GDP \\ $\boldsymbol{h=2Q}$}} &
\multicolumn{2}{c}{\makecell{\bfseries Unemployment \\ \bfseries $\boldsymbol{h=2Q}$}} &
\multicolumn{2}{c}{\makecell{\bfseries house price index \\ \bfseries $\boldsymbol{h=2Q}$}} &
\multicolumn{2}{c}{\makecell{\bfseries Inflation \\ \bfseries $\boldsymbol{h=2Q}$}} \\
\cmidrule(lr){2-3} \cmidrule(lr){4-5} \cmidrule(lr){6-7} \cmidrule(lr){8-9}
& \multicolumn{1}{c}{\bfseries Mean} & \multicolumn{1}{c}{\bfseries RMSE} & \multicolumn{1}{c}{\bfseries Mean} & \multicolumn{1}{c}{\bfseries RMSE} & \multicolumn{1}{c}{\bfseries Mean} & \multicolumn{1}{c}{\bfseries RMSE} & \multicolumn{1}{c}{\bfseries Mean} & \multicolumn{1}{c}{\bfseries RMSE} \\
\midrule
$b_k = 0$ & & & & & & & & \\
\hspace{7pt} $\beta$ & 0.097 & 0.009& 0.245 & 0.009 & 0.013 & 0.089 & 0.003& 0.078 \\
\hspace{7pt} $\sigma$ & 0.003 & 0.013 & 0.077& 0.015 & 0.065 & 0.002 & 0.074& 0.000\\
\hspace{7pt} $\theta$ & 0.184 & 0.045 & 0.069 & 0.078 & 0.005 & 0.046 & 0.078 & 0.055 \\
\hspace{7pt} $\delta$ & 0.165 & 0.036 & 0.078 & 0.004 & 0.063& 0.015 & 0.045 & 0.077\\
\addlinespace
$b_k \neq 0$ & & & & & & & & \\
\hspace{7pt} $\beta$ & 0.002 & 0.889 & 0.764 & 0.445 & 0.057 & 0.447 & 0.008 & 0.078 \\
\hspace{7pt} $\sigma$ & 0.001 & 0.446 & 0.003 & 0.096 & 0.065 & 0.047 & 0.005 & 0.054\\
\hspace{7pt} $\theta$ & 0.004 & 0.410 & 0.004 & 0.080 & 0.045 & 0.004 & 0.014& 0.036\\
\hspace{7pt} $\delta$ & 0.005 & 0.780 & 0.002 & 0.070 & 0.052 & 0.046 & 0.051 & 0.065 \\
\bottomrule
\end{tabular}
\end{sidewaystable}
However, this is the output:
MWE
from\documentclass{...}
to\end{document}