documentclass{scrartcl}
usepackage{array}
usepackage{ragged2e}
usepackage{booktabs}
usepackage{threeparttable}
usepackage{siunitx}
usepackage{xparse}
usepackage{float}

floatstyle{komaabove}
restylefloat{table}
newcolumntype{L}[1]{>{RaggedRighthspace{0pt}}p{dimexpr #1linewidth-2tabcolsep}}
newcolumntype{C}[1]{>{Centeringhspace{0pt}}p{dimexpr #1linewidth-2tabcolsep}}
newcommand{uhrule}{midrule[heavyrulewidth]}

DeclareExpandableDocumentCommand{mcc}{o o m}{%
    IfNoValueTF{#1}
    {multicolumn{1}{c}{#3}}
        {IfNoValueTF{#2}
            {multicolumn{#1}{c}{#3}}
            {multicolumn{#1}{C{#2}}{#3}}
    }
}

begin{document}
begin{table}[t]
centering
begin{threeparttable}
begin{tabular}{L{0.15}
            S[table-format=6]
            S[table-format=5]
            S[table-format=2.1,
              table-space-text-post = {**}]
           }
toprule
Race
& mcc[1][0.20]{Populationtnote{b}}
& mcc[1][0.20]{Homicides}
& mcc[1][0.10]{Ratetnote{c}} \ uhrule

Blacktnote{d} & 29986   & 12114 & 40.5* \
White          & 199686  & 12153 &  6.1  \
Othertnote{e} & 19038   & 635   &  3.3** \ midrule
Total          & 248710  & 24932 & 10.0 \
bottomrule
end{tabular}
begin{tablenotes}[flushleft]
item[a] Data developped from the Statistical Abstract of the United States -- 1993.
item[b] Population in $1000$s.
item[c] Rate per $100,000$ in the population.
item[d] The rate for emph{Black} males was 69.2, for emph{White} males it was 9.0.
item[e] About one-half of the emph{Other} category was composed of Asian Americans and Native Americans.
end{tablenotes}
$*p<.001$, two-tailed test. $**p<0.05$, two-tailed test.
caption{Homicides by race of the victim in 1993 in the USA.tnote{a}}
label{tab5}
end{threeparttable}
end{table}
end{document}

documentclass{scrartcl}
usepackage{array}
usepackage{ragged2e}
usepackage{booktabs}
usepackage{threeparttable}
usepackage{siunitx}
usepackage{xparse}
usepackage{float}

floatstyle{komaabove}
restylefloat{table}
newcolumntype{L}[1]{>{RaggedRighthspace{0pt}}p{dimexpr #1linewidth-2tabcolsep}}
newcolumntype{C}[1]{>{Centeringhspace{0pt}}p{dimexpr #1linewidth-2tabcolsep}}
newcommand{uhrule}{midrule[heavyrulewidth]}

DeclareExpandableDocumentCommand{mcc}{o o m}{%
    IfNoValueTF{#1}
    {multicolumn{1}{c}{#3}}
        {IfNoValueTF{#2}
            {multicolumn{#1}{c}{#3}}
            {multicolumn{#1}{C{#2}}{#3}}
    }
}

begin{document}
begin{table}[t]
centering
begin{threeparttable}
caption{Homicides by race of the victim in 1993 in the USA.tnote{a}}
label{tab5}

begin{tabular}{L{0.15}
            S[table-format=6]
            S[table-format=5]
            S[table-format=2.1,
              table-space-text-post = {**}]
           }
toprule
Race
& mcc[1][0.20]{Populationtnote{b}}
& mcc[1][0.20]{Homicides}
& mcc[1][0.10]{Ratetnote{c}} \ uhrule

Blacktnote{d} & 29986   & 12114 & 40.5* \
White          & 199686  & 12153 &  6.1  \
Othertnote{e} & 19038   & 635   &  3.3** \ midrule
Total          & 248710  & 24932 & 10.0 \
bottomrule
end{tabular}
begin{tablenotes}[flushleft]
item[a] Data developped from the Statistical Abstract of the United States -- 1993.
item[b] Population in $1000$s.
item[c] Rate per $100,000$ in the population.
item[d] The rate for emph{Black} males was 69.2, for emph{White} males it was 9.0.
item[e] About one-half of the emph{Other} category was composed of Asian Americans and Native Americans.
end{tablenotes}
$*p<.001$, two-tailed test. $**p<0.05$, two-tailed test.
end{threeparttable}
end{table}
end{document}