Q:

centering a line in table's cell

I'm using two columns style. I would like to modify a table so it can fit with keeping clarity. Two of the cells their texts should be in two lines. The second line should be in the center. 
\documentclass[11pt,twocolumn]{article}
\usepackage[margin=1cm]{geometry}
\usepackage{amsmath}
\usepackage{pbox}
\begin{document}

\begin{table}[h]
\caption{An Example of a Table}
\label{table_example}
\begin{center}
\begin{tabular}{|c|c|c|c|c|c|c|c|}
\hline
A  & B & C & D & Q & W &  error $\%$  in x  &  \pbox{20cm}{error $\%$ \\ in x } \\
\hline
23 & 1   & 80.9281   & 80.9281   &  80.9281          & aa                &      aa &\\
\hline
23 & 1   & 80.9281   & 80.9281   &  80.9281          & aa                &      aa &\\
\hline
23 & 1   & 80.9281   & 80.9281   &  80.9281          & aa                &      aa &\\
\hline
23 & 1   & 80.9281   & 80.9281   &  80.9281          & aa                &      aa &\\
\hline
23 & 3   & 80.9281   & 80.9281   &  80.9281          & aa                &      aa &\\
\hline
\end{tabular}
\end{center}
\end{table}

\end{document}

This is the picture of the code 

A:

You should use the makecell package for that. It's dedicated to common formatting of multiline cells and defines a \thead and a \makecell command a multirow version of these. The default alignment in such cells is centred (vertically and horizontally) with keywords to choose among t, b, c and l, r, c, independently of the column specifier. Here are two examples using \thead, for which I specify to use a boldface font. 
Also I added the caption package to have a proper vertical spacing between caption and table, and the cellspace package to ensure  a minimal spacing between the top of a cell and the line above, and between the bottom of a cell  and the line below; it works defining an S pre-specifier for columns. 
Another solution to get soacing between rows is to use the booktabs package; it defines new commands for horizontal rules of variable thickness, with some vertical spacing above and below; it this spacing is not enough, you have the \addlinespace command.
Finally I replaced the center environment with centering (center adds unwanted here vertical spacing):
\documentclass[11pt,twocolumn]{article}
\usepackage[margin=1cm,]{geometry}
\usepackage{amsmath}
\usepackage{pbox}
\usepackage{caption, booktabs}
\usepackage{makecell}
\usepackage{cellspace}
\setlength\cellspacetoplimit{5pt}
\setlength\cellspacebottomlimit{5pt}

\renewcommand\theadfont{\bfseries}

\begin{document}

\begin{table}[h]
\caption{An Example of a Table}
\centering\label{table_example}
\begin{tabular}{|Sc*{8}{c|}}
\hline
\thead{A} &\thead{ B} & \thead{C} & \thead{D} & \thead{Q} & \thead{W} & \thead{error \% in x} & \thead{error \% \\ in x } \\
\hline
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
23 & 3 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
\end{tabular}
\end{table}

\begin{table}[h]
\renewcommand\theadalign{bc}
\caption{An Example of a Table}
\centering\label{table_example}
\begin{tabular}{|*{8}{c|}}
\hline
\thead{A} &\thead{ B} & \thead{C} & \thead{D} & \thead{Q} & \thead{W} & \thead{error \% in x} & \thead{error \% \\ in x } \\
\hline
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
23 & 3 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\hline
\end{tabular}
\end{table}

\begin{table}[h]
\renewcommand\theadalign{bc}
\caption{An Example of a Table}
\centering\label{table_example}
\begin{tabular}{*{8}{c}}
\toprule
\thead{A} &\thead{ B} & \thead{C} & \thead{D} & \thead{Q} & \thead{W} & \thead{error \% in x} & \thead{error \% \\ in x } \\
\cmidrule(lr){1-8}
\addlinespace
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\addlinespace
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\addlinespace
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\addlinespace
23 & 1 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\addlinespace
23 & 3 & 80.9281 & 80.9281 & 80.9281 & aa & aa &\\
\bottomrule
\end{tabular}
\end{table}

\end{document} 

