sidewaystable with 8 coloumns

Question

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:

Answer 1

Your question is not very clear. You already have filled 8 columns and have a header for two more, yet undefined columns. Are this means that you like to have two more? Anyway, you just define number of columns which you like to have and add their data in them:

\documentclass{article}
\usepackage{bm}         % new
\usepackage{rotating}
\usepackage{siunitx}
\usepackage{booktabs, makecell, multirow, threeparttable}
\usepackage{caption}

\usepackage{xparse}     % new
\NewExpandableDocumentCommand\mcc{O{1}m}
    {\multicolumn{#1}{>{\bfseries}c}{#2}}

\begin{document}
    \begin{sidewaystable}
\centering
\caption{Realtime forecast RMSEs for $\bm{h=2Q}$}
\label{forecast}
\begin{threeparttable}
\begin{tabular}{@{}
             >{$}c< {$}
           *{12}{S[table-format=1.3]}   %   <--
                @{}}
    \toprule
\multirow{2.4}{3em}{\bfseries Para- meter} 
    & \mcc[2]{$\bm{S\&P$}\tnote{a}}    
                 & \mcc[2]{GDP\tnote{b}}         
                              & \mcc[2]{Unemploy.}
                                            & \mcc[2]{HPI\tnote{c}}                  \\  
    \cmidrule(l){2-3} \cmidrule(l){4-5}     \cmidrule(l){6-7} 
    \cmidrule(l){8-9} \cmidrule(l){10-11}   \cmidrule(l){12-13}                    
    & \mcc{Mean} & \mcc{RMSE} & \mcc{Mean} & \mcc{RMSE} & \mcc{Mean} & \mcc{RMSE} 
    & \mcc{Mean} & \mcc{RMSE} & \mcc{Mean} & \mcc{RMSE} & \mcc{Mean} & \mcc{RMSE}   \\
    \midrule
b_k = 0 & \mcc[10]{}                                                                \\
\beta   & 0.097  & 0.009 & 0.245 & 0.009 & 0.013 & 0.089 & 0.003 & 0.078 &  &  &  & \\
\sigma  & 0.003  & 0.013 & 0.077 & 0.015 & 0.065 & 0.002 & 0.074 & 0.000 &  &  &  & \\
\theta  & 0.184  & 0.045 & 0.069 & 0.078 & 0.005 & 0.046 & 0.078 & 0.055 &  &  &  & \\
\delta  & 0.165  & 0.036 & 0.078 & 0.004 & 0.063 & 0.015 & 0.045 & 0.077 &  &  &  & \\
\addlinespace
b_k\neq 0   & \mcc[12]{}                                                            \\
\beta   & 0.002  & 0.889 & 0.764 & 0.445 & 0.057 & 0.447 & 0.008 & 0.078 &  &  &  & \\
\sigma  & 0.001  & 0.446 & 0.003 & 0.096 & 0.065 & 0.047 & 0.005 & 0.054 &  &  &  & \\
\theta  & 0.004  & 0.410 & 0.004 & 0.080 & 0.045 & 0.004 & 0.014 & 0.036 &  &  &  & \\
\delta  & 0.005  & 0.780 & 0.002 & 0.070 & 0.052 & 0.046 & 0.051 & 0.065 &  &  &  & \\
    \bottomrule
\end{tabular}
\begin{tablenotes}[para, flushleft]
\item[a] S\&P ...,
\item[b] GDP ...,
\item[c] Hause Price Index.
\end{tablenotes}
\end{threeparttable}
    \end{sidewaystable}
\end{document}

which gives:

In above code I redefine S columns, add threeparttable for use oftablenotesmacro\tnote` for explanation of meaning of shorteness in column headers.