How can I draw a table where the upper left corner is missing? (Pic)

Question

Is it possible to draw a table like the one in the attached picture? I've read documentation on the tabular environment but nothing really describes this well. The shading would be nice but the crucial part is the layout (5 columns, 2x2 merged cells on the top row and the top left 2 cells have no shading or border)

Answer 1

colortbl can add the grey:

\documentclass{article}

\usepackage{colortbl,hhline}

\setlength\extrarowheight{5pt}
\definecolor{g}{rgb}{.8,.8,.8}
\begin{document}

$
\begin{array}{|*5{c|}}
\hhline{~----}
\multicolumn{1}{c|}{}&\multicolumn{2}{c|}{\cellcolor{g}c}&\multicolumn{2}{c|}{\cellcolor{g}\lnot c }\\
\hhline{~----}
\multicolumn{1}{c|}{}&\cellcolor{g}b&\cellcolor{g}\lnot b&\cellcolor{g}b&\cellcolor{g}\lnot b\\
\hline
\cellcolor{g}a&&0.33&0.03&\\
\hline
\cellcolor{g}\lnot a&&0.17&0.01&\\
\hline
\end{array}
$

\end{document}

Answer 2

This can be done using the \multicolumn which has syntax

\multicolumn{<number of cells to span>}{<justification>}{<content>}

and the \cline command which takes arguments

 \cline{<start column>-<end column>}

MWE

\documentclass{article}
\begin{document}

\noindent\begin{tabular}{|c|c|c|c|c|}
  \cline{2-5}
    \multicolumn{1}{c}{}  &   \multicolumn{2}{|c|}{c} &   \multicolumn{2}{|c|}{c}\\
  \cline{2-5}
  \multicolumn{1}{c}{}  &   \multicolumn{1}{|c|}{b}   &   b   &   b   &   b\\\hline
   a &      &   $.33$   &   $0.03$   &   \\\hline
   a &      &   $.33$   &   $0.03$   &   \\\hline
\end{tabular}
\end{document}

Answer 3

The following solution uses \multicolumn and \cline to get the empty cells without lines. It solves the problem of colortbl that the cell background color overprints the lines by setting the table twice, the first time with background color, the second time without.

\documentclass{article}
\usepackage{array}
\usepackage{colortbl}
\definecolor{tabgray}{gray}{.8}

\renewcommand*{\arraystretch}{1.2}

\begin{document}
\newcommand*{\MyTable}[1]{%
  \begin{tabular}{|>{#1}c|c|c|c|c|}
    \cline{2-5}
    \multicolumn{1}{l|}{} &
    \multicolumn{2}{>{#1}c|}{c} &
    \multicolumn{2}{>{#1}c|}{$\lnot$c} \\
    \cline{2-5}
    \multicolumn{1}{l|}{} &
    % b & $\lnot$b & b & $\lnot$b \\
    \multicolumn{1}{>{#1}c|}{b} &
    \multicolumn{1}{>{#1}c|}{$\lnot$b} &
    \multicolumn{1}{>{#1}c|}{b} &
    \multicolumn{1}{>{#1}c|}{$\lnot$b} \\
    \hline
    a & & 0.33 & 0.03 & \\
    \hline
    $\lnot$a & & 0.17 & 0.01 & \\
    \hline
  \end{tabular}%
}%
\leavevmode
\rlap{\MyTable{\columncolor{tabgray}}}%
\mbox{\MyTable{}}%
\end{document}