\begin{Schunk}
\begin{Sinput}
 table(dat_nat$ID, dat_nat$Quarter, dat_nat$Year)
\end{Sinput}
\begin{Soutput}
, ,  = 1990

     
      Q1 Q2
  UK   1  1
  USA  1  1

, ,  = 1991

     
      Q1 Q2
  UK   1  1
  USA  1  1
\end{Soutput}
\end{Schunk}
