compareGroups table with knitr - how to adjust table head and subtext?

Question

I just discovered the wonderful package compareGroups and I think this may help me a lot in the future. So far, it is working brilliantly in combination with Knitr/Sweave:

\documentclass{article}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% LaTeX packages needed for compareGroups

% long tables
\usepackage{longtable}

% multi row
\usepackage{multirow}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}

% R code
<<results = "asis">>=

library(compareGroups)

# Very short code for bivariate descriptive table
tab <- compareGroups(vs ~ . , data = mtcars)
export2latex(createTable(tab))

@

\end{document}

produces

I have only two concerns which is i) the modification of the table head (e.g. I want to print "P value" instead of "p.overall") and ii) adding a multicolumn subtext (e.g. "t-test was used to calculate P values."). If I just exported the table to LaTeX Code, it would be easy to modify head and subtext but in combination with Knitr I don't know how to deal with it. I just really would appreciate this wonderful solution over laborious functions in combination with e.g. Xtables package.

Thanks so much for your help!

Answer 1

Concern i) can be dealt with by manually reassigning the name of the 3rd column of matrix tab$descr prior to calling export2latex. Concern ii) might be addressed alternatively by placing the text in a caption (it seems to me more appropriate -- and further I have no idea about how to stick that into a multicol subheader).

tab <- compareGroups(vs ~ . , data = mtcars)
tab <- createTable(tab)
colnames(tab$descr)[3] <- "P value"
export2latex(tab, caption="$t$-test was used to calculate $p$ values.",
             loc.caption="bottom")

Regarding your request (below) to add, in fact, two captions (a numbered caption above the table and a non-numbered explanatory sub-text below the table), one way to achieve this is to encapsulate the longtable produced by export2latex within a (possibly centered) minipage environment, and placing the second caption as ordinary text below the table:

\documentclass{article}
\usepackage{longtable}
\usepackage{multirow}

\begin{document}

See Table~\ref{tab:complicated} for the main results of this paper.
\begin{center}
  \begin{minipage}[c]{0.5\textwidth}

<<results = "asis", echo=FALSE>>=

library(compareGroups)

tab <- compareGroups(vs ~ . , data = mtcars)
tab <- createTable(tab)
colnames(tab$descr)[3] <- "P value"
export2latex(tab, caption="Characteristics of the patients at baseline$^*$.",
             label="tab:complicated"
             )
@
{\footnotesize $^*$A two-sided $t$-test was used to calculate the $p$ values.}
\end{minipage} 
\end{center}

\end{document}

References:

https://tex.stackexchange.com/questions/15282/tabular-title-above-and-caption-below