How to make a \cline or \cmidrule narrower?

Question

I'm trying to copy this table into TeX:

I have problems with the horizontal lines under "Gaussian" etc. When I specify three \cmidrule's, they are not separated but are combined into one big \midrule or \cline.

It would be great if someone could show me how to narrow down such a \cmidrule. Thanks in advance!

Here's the (spaghetti) code I use:

\documentclass[bibtotoc,liststotoc,BCOR5mm,DIV12]{scrbook} 
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{colortbl,array} % für farbige cells
\usepackage{multirow,bigdelim}
\usepackage{booktabs}

\begin{document}

\begin{table}[htb]
\resizebox{\textwidth}{!}{
\begin{tabular}{lr rrrrrr rrrrrr rrrr}
\toprule
$f$ & $n$ & \multicolumn{6}{l}{Gaussian} &  \multicolumn{6}{l}{Binomial Logit} & \multicolumn{4}{l}{Survival} \\ \cmidrule{3-8} \cmidrule{9-14} \cmidrule{15-18}
 & & \multicolumn{2}{l}{FP}  & \multicolumn{2}{l}{GCV} & \multicolumn{2}{l}{REML} & \multicolumn{2}{l}{FP} & \multicolumn{2}{l}{GCV} & \multicolumn{2}{l}{REML} & \multicolumn{2}{l}{FP} & AIC & REML \\ 
 &  & \makebox[1cm][l]{FP2}
 & \makebox[1cm][l]{FP4} & \makebox[1cm][l]{LRSS} & \makebox[1cm][l]{PS} & \makebox[1cm][l]{LRSS} & \makebox[1cm][l]{PS} & \makebox[1cm][l]{FP2} & \makebox[1cm][l]{FP4} & \makebox[1cm][l]{LRSS} & \makebox[1cm][l]{PS} & \makebox[1cm][l]{LRSS}
 & \makebox[1cm][l]{PS} & \makebox[1cm][l]{FP2} & \makebox[1cm][l]{FP4} & \makebox[1cm][l]{PS} & \makebox[1cm][l]{PS} \\ \midrule
 & 100 & \cellcolor[gray]{0.8} 0.012 & \cellcolor[gray]{0.8} 0.012 & 0.021 & 0.025 & 0.018 & 0.018 & 0.043 & \cellcolor[gray]{0.8} 0.042 & 0.146 & 0.194 & 0.074 & 0.076 & \cellcolor[gray]{0.8} 0.015 & 0.016 & 0.427 & 0.019 \\ 
$f_1$ & 500 & 0.004 & \cellcolor[gray]{0.8} 0.002 & 0.006 & 0.006 & 0.004 & 0.004 & 0.014 & \cellcolor[gray]{0.8} 0.011 & 0.030 & 0.034 & 0.028 & 0.028 & 0.020 & 0.007 & 0.024 & \cellcolor[gray]{0.8} 0.005

\end{tabular}
}
\end{table}

\end{document}

Answer 1

The \cmidrule command of booktabs allows for an optional argument using parentheses ( ) to specify on which side it should be reduced:

\cmidrule(r){3-8} \cmidrule(r){9-14} \cmidrule{15-18}

would shorten the right end of the first two rules. You can also add an amount in braces, i.e. (r{10pt}) and select both sides (lr), (l{5pt}r{10pt}). See the description of \cmidrule in the booktabs manual on page 4 for more details.