\begin{Schunk}
\begin{Sinput}
 dat_nat
\end{Sinput}
\begin{Soutput}
   ID Year Quarter population illnesses
1 USA 1990      Q1  10.585529  97.15840
2 USA 1990      Q2  10.709466  90.80678
3 USA 1991      Q1   9.890697  98.83752
4 USA 1991      Q2   9.546503 118.17312
5  UK 1990      Q1  10.605887 103.70628
6  UK 1990      Q2   8.182044 105.20216
7  UK 1991      Q1  10.630099  92.49468
8  UK 1991      Q2   9.723816 108.16900
\end{Soutput}
\end{Schunk}
