documentclass[11pt]{article}
usepackage{multirow}
begin{document}
 % Text data placeholder. May be long enough that it needs to be wrapped.
begin{table}[ht]
    begin{center}
        begin{tabular}{cccc}
            hline
            multirow{2}{*}{Outer index 1} & Inner index 1.1 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            & Inner index 1.2 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            hline
            multirow{2}{*}{Outer index 2} & Inner index 2.1 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            & Inner index 2.2 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            hline
            multirow{6}{*}{Outer index 3} &  Inner index 3.1 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            &Inner index 3.2 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            &Inner index 3.3 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            &Inner index 3.4 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            &Inner index 3.5 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            &Inner index 3.5 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            &Inner index 3.6 & Text data placeholder. May be long enough that it needs to be wrapped. & Text data placeholder. May be long enough that it needs to be wrapped.\
            hline
        end{tabular}
    end{center}
end{table}

end{document}