I would like to put "U.S. census" in the middle of the given space, and also "GroupA" as well. Currently it's somewhat shifted. How should I combine the rows?
\begin{table} \begin{tabular}{lrrrrrr} \toprule & U.S. Census & \multicolumn{5}{c}{Study2} \\ & & GroupA & \multicolumn{4}{c}{GroupB} \\ & & & A & B & C & D \\ \bottomrule \end{tabular} \end{table} 
I tried multirow and makecell functions but both didn't work. Thanks in advance.

