Shrinking vertical space when using multirow

Question

When using multirow, how can I shrink the space between the two rows while still keeping everything vertically centered? Here's an example

\documentclass{article}
\usepackage{multirow}
% ---------------
\begin{document}
\begin{tabular}{l l}
some & text \\
& b \\[-4pt]
\multirow{-2}{*}{a}
& b \\
some & text \\
\end{tabular}
% ----------------
\begin{tabular}{l l}
some & text \\
& b \\
\multirow{-2}{*}{a}
& b \\
some & text \\
\end{tabular}
\end{document}

If I shrink the space between the rows with the two b's, the a isn't vertically centered anymore.

Answer 1

You do not need any packages to achieve this, you can just nest tabulars.

\documentclass{article}
\begin{document}
\begin{tabular}{l l}
some & text \\
a & \begin{tabular}{@{}l}b \\[-4pt] b
\end{tabular}\\
some & text \\
\end{tabular}
\end{document}

Comparison:

\documentclass{article}
\usepackage{multirow}
% ---------------
\begin{document}
\begin{tabular}{l l}
some & text \\
a & \begin{tabular}{@{}l}b \\[-4pt] b
\end{tabular}\\
some & text \\
\end{tabular}
% ----------------
\begin{tabular}{l l}
some & text \\
& b \\[-4pt]
\multirow{-2}{*}{a}
& b \\
some & text \\
\end{tabular}
% ----------------
\begin{tabular}{l l}
some & text \\
& b \\
\multirow{-2}{*}{a}
& b \\
some & text \\
\end{tabular}
\end{document}

Answer 2

Would using \makecell, without multirow, be enough for your needs? By defult, its content is vertically and horizontally centred.

\documentclass{article}
\usepackage{makecell}
\renewcommand{\cellset}{\renewcommand{\arraystretch}{0.5}}

\begin{document}

\begin{tabular}{l l}
some & text \\
a & \makecell{ b \\ b }\\
some & text \\
\end{tabular}

\end{document}