documentclass[11pt]{article}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage[english]{babel}
usepackage[margin=2.5cm]{geometry} % showframe
usepackage{multirow}
usepackage{multicol}
usepackage{tabularx}
usepackage{caption}
usepackage{booktabs}
usepackage{amsmath}
usepackage{mathtools}
usepackage{siunitx}
usepackage[skip=0.33baselineskip]{caption}

captionsetup[table]{
    font=bf,
    size=normalsize,
    justification=centerlast,
    labelsep=newline
    }
renewcommandthetable{Roman{table}}

begin{document}
defarraystretch{1.5}
begin{table}[h]
centering
setlengthtabcolsep{0pt}
sisetup{table-format=2.2, group-separator={,}, group-minimum-digits=4}
caption{large{Header}}
label{table:fundsummary}
parbox{linewidth}{small{This table presents some very interesting and very fancy data, which I expect will provide me with at least two Nobel prizes.}vspace{2ex}}
medskipsmallbegin{tabular*}{linewidth}{@{extracolsep{fill}}
                l
                S[table-format=-2.2]
           *{7}{S}
                }
toprule
multicolumn{1}{c}{multirow{2}{*}{Description}}&
multicolumn{1}{c}{multirow{2}{*}{Variable}}&
multicolumn{5}{c}{Averages}\cmidrule{3-7}
multicolumn{2}{c}{}&{p-value}&{Factor 1}&{Factor 2}&{Factor 3}&{Factor 4}\midrule
Section 1.1     &11.11 &22.22{***} &33.33   &44.44   &55.55  &66.66\midrule
Section 1.2     &11.11 &(22.22){***} &33.33   &44.44   &55.55  &66.66\
Section 1.3     &11.11 &22.22{***} &33.33   &44.44   &55.55  &66.66\
Section 1.4     &11.11 &(22.22){***} &33.33   &44.44   &55.55  &66.66\
bottomrule
end{tabular*}
end{table}
end{document}