Ah ok, but where would that be in the following code (sorry for the 'newbie question', but this is literally the first time I'm using a longtable, and the code below was produced by someone else for me):
\noindent\begin{longtable}{>{\raggedright\arraybackslash}p{.17\textwidth}*{5}{S}}% don't use vertical rules with booktabs (see the documentation)
\caption {Descriptive Statistics (\%): Commerce Faculty ($N = \num{7028}$)}
\toprule
% define \endfirsthead if first head differs from later heads
{\bfseries\hskip0pt Variable} & \multicolumn{1}{p{.17\textwidth}}{\bfseries\hskip0pt Proportion of …