Vertical alignment in multirow environment

Question

I'm using a multirow environment to span text over two rows. As it does not support \\ for inserting a line break, I'm nesting it into a shortstack (see MWE below). However, the text is in neither of both cases (single line text, dual line text) really centered. What am I doing wrong?

Example Image

The red line indicates the center of the heading row (spans over two rows). As shown in the image, neither of both lines are really vertically centered.

MWE

\documentclass[a4paper,twocolumn]{article}

\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{booktabs}
\usepackage{multirow}

\usepackage[a4paper,top=3cm,bottom=2cm,left=1.5cm,right=1.5cm,marginparwidth=1.75cm]{geometry}

\title{Your Paper}
\author{You}

\begin{document}
    \begin{table}[htbp]
        \caption{Classifier configurations and their performance.}
        \label{tblClassifierConfig}
        \begin{tabular*}{1.2\columnwidth}{@{\extracolsep{\fill}}lrllll@{}}
        \toprule
        \multirow{2}{*}{\raisebox{-\heavyrulewidth}{\shortstack[l]{\textbf{N-gram}\\as Range}}} & \multirow{2}{*}{\raisebox{-\heavyrulewidth}{\shortstack[l]{\textbf{Vocab. Size}\\in \#n-grams}}} & \multicolumn{2}{c}{Test Set 1} & \multicolumn{2}{c}{Test Set 2} \\ 
        \cmidrule(l){3-6} &  & \textbf{Prec.} & \textbf{Recall} & \textbf{Prec.} & \textbf{Recall} \\ \midrule
        (2,4)-char                        & 500                                 & 0.9999         & 0.9999          & 0.5085         & 0.5849          \\
        (2,4)-char                        & 1000                                & 0.9999         & 0.9999          & 0.5114         & 0.6132          \\
        (2,3)-word                        & 1,500                               & 0.9999         & 0.9999          & 0.5134         & 0.6268          \\
        (1,3)-word                        & 2,000                               & 0.9999         & 0.9999          & 0.5138         & 0.6307          \\ \bottomrule
        \end{tabular*}
    \end{table}
\end{document}

Answer 1

in multirow cells you also need to consider vertical space introduced by \cmidrule. it is about 0.4 row height, so you can wrote

 \multirow{2.4}{*}{ ...}

i also suggest in columns with numbers to use S column type from siunitx package, not increase table width (it will protrude in next column). istead this rather reduce tabocolsep:

\documentclass[a4paper,twocolumn,english]{article}

\usepackage{babel}
\usepackage[T1]{fontenc}
\usepackage{booktabs, makecell, multirow}
\usepackage{siunitx}
\usepackage[a4paper,
            top=3cm,bottom=2cm,left=1.5cm,right=1.5cm,
             marginparwidth=1.75cm]{geometry}
\usepackage{lipsum}

\title{Your Paper}
\author{You}

\begin{document}
\lipsum[11]
\begin{table}[htbp]
    \caption{Classifier configurations and their performance.}
    \label{tblClassifierConfig}
    \setlength\tabcolsep{0pt}
    \begin{tabular*}{\columnwidth}{@{\extracolsep{\fill}}
                            l
                             S[table-format=4.0,
                               group-four-digits,
                               group-separator=\,]
                        *{4}{S[table-format=1.4]}
                                   }
    \toprule
    \multirow{2.4}{*}{\makecell{\textbf{N-gram}\\as Range}} 
        & {\multirow{2.4}{*}{\makecell{\textbf{Vocab. Size}\\in \#n-grams}}}
            & \multicolumn{2}{c}{Test Set 1} 
                & \multicolumn{2}{c}{Test Set 2} \\
    \cmidrule{3-4} 
    \cmidrule{5-6}
        &  & {\textbf{Prec.}} & {\textbf{Recall}} & {\textbf{Prec.}} & {\textbf{Recall}} \\ 
    \midrule
    (2,4)-char  &   500 & 0.9999    & 0.9999    & 0.5085    & 0.5849            \\
    (2,4)-char  & 1 000 & 0.9999    & 0.9999    & 0.5114    & 0.6132            \\
    (2,3)-word  & 1 500 & 0.9999    & 0.9999    & 0.5134    & 0.6268            \\
    (1,3)-word  & 2 000 & 0.9999    & 0.9999    & 0.5138    & 0.6307            \\ 
    \bottomrule
    \end{tabular*}
\end{table}
\end{document}

which gives

Answer 2

A variant with \multirowcell. The makecell package allows for line breaks in standard cells, and a common formatting (not used here). Please note your table is larger than the column width, so it will overflow into the column on the right. I loaded caption to have a sensible vertical spacing between caption and table. Also, I wonder what a \marginparwidth of 1.75cm means when the horizontal margins are set to 1.5cm.

\documentclass[a4paper,twocolumn]{article}

\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{multirow}
\usepackage{booktabs, makecell, caption}
\captionsetup{skip=6pt}
\usepackage[a4paper,top=3cm, bottom=2cm, hmargin=1.5cm,marginparwidth=1.75cm, showframe]{geometry}

\usepackage{lipsum}
\title{Your Paper}
\author{You}

\begin{document}
\lipsum[11]

    \begin{table}[!htbp]
        \caption{Classifier configurations and their performance.}
        \label{tblClassifierConfig}
        \begin{tabular*}{1.2\columnwidth}{@{\extracolsep{\fill}}lrllll@{}}
        \toprule
        \multirowcell{2.3}{\textbf{N-gram}\\as Range} & \multirowcell{2.3}{\textbf{Vocab. Size}\\in \#n-grams} & \multicolumn{2}{c}{Test Set 1} & \multicolumn{2}{c}{Test Set 2} \\
        \cmidrule(l){3-6} & & \textbf{Prec.} & \textbf{Recall} & \textbf{Prec.} & \textbf{Recall} \\ \midrule
        (2,4)-char & 500 & 0.9999 & 0.9999 & 0.5085 & 0.5849 \\
        (2,4)-char & 1000 & 0.9999 & 0.9999 & 0.5114 & 0.6132 \\
        (2,3)-word & 1,500 & 0.9999 & 0.9999 & 0.5134 & 0.6268 \\
        (1,3)-word & 2,000 & 0.9999 & 0.9999 & 0.5138 & 0.6307 \\ \bottomrule
        \end{tabular*}
    \end{table}

\end{document} 

Answer 3

Using stackengine, I change \shortstack to a long \stackanchor, but (importantly) also added a 4pt vertical buffer above and below the stack.

\documentclass[a4paper,twocolumn]{article}

\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{booktabs}
\usepackage{multirow}
\usepackage[usestackEOL]{stackengine}
\usepackage[a4paper,top=3cm,bottom=2cm,left=1.5cm,right=1.5cm,marginparwidth=1.75cm]{geometry}
\def\stacktype{L}
\setstackgap{S}{4pt}
\title{Your Paper}
\author{You}

\begin{document}
    \begin{table}[htbp]
        \caption{Classifier configurations and their performance.}
        \label{tblClassifierConfig}
        \begin{tabular*}{1.2\columnwidth}{@{\extracolsep{\fill}}lrllll@{}}
        \toprule
        \multirow{2}{*}{\raisebox{-\heavyrulewidth}{\addstackgap{\stackanchor{\textbf{N-gram}}{as Range}}}} & \multirow{2}{*}{\raisebox{-\heavyrulewidth}{\addstackgap{\stackanchor{\textbf{Vocab. Size}}{in \#n-grams}}}} & \multicolumn{2}{c}{Test Set 1} & \multicolumn{2}{c}{Test Set 2} \\ 
        \cmidrule(l){3-6} &  & \textbf{Prec.} & \textbf{Recall} & \textbf{Prec.} & \textbf{Recall} \\ \midrule
        (2,4)-char                        & 500                                 & 0.9999         & 0.9999          & 0.5085         & 0.5849          \\
        (2,4)-char                        & 1000                                & 0.9999         & 0.9999          & 0.5114         & 0.6132          \\
        (2,3)-word                        & 1,500                               & 0.9999         & 0.9999          & 0.5134         & 0.6268          \\
        (1,3)-word                        & 2,000                               & 0.9999         & 0.9999          & 0.5138         & 0.6307          \\ \bottomrule
        \end{tabular*}
    \end{table}
\end{document}