How to add a background to a flow chart with tikz?

Question

Background:

I am trying to add a background to my flow chart and group them into two, namely Quantitative Analysis and Theoretical Analysis.

Relevant Research:

I have tried to understand and modify the codes from:

• Example: Schema of Labs on a class
• Example: System Combination

but couldn't get it right.

Working Code:

\documentclass[a4paper, 11pt]{report}
\usepackage[toc,page]{appendix}
\usepackage{pgfgantt}
\usepackage{geometry}

\geometry{
 a4paper,
 total={170mm,257mm},
 left=20mm,
 top=20mm,
 }

\usepackage{float}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric, arrows, shadows}

\begin{document}

\tikzstyle{rect} = [rectangle, rounded corners, minimum width=4.5cm, minimum height=1cm,text centered, draw=black, fill=blue!10, drop shadow]
\tikzstyle{arrow} = [thick,->,>=stealth]

\begin{figure}[H]
\begin{center}
\begin{tikzpicture}[node distance=2cm]

\node (intro) [rect] {Introduction};
\node (method) [rect, below right = 0.75cm and -1.1cm  of intro] {Methodology \& Model};
\node (litrev) [rect, below left = 0.75cm and -1.1cm  of intro] {Literature Review};
\node (result) [rect, below right = 2.50cm and -1.1cm  of intro] {Results};
\node (conc) [rect, below= 4.250cm of intro] {Conclusion};

\draw [arrow] (intro) -| (method);
\draw [arrow] (intro) -| (litrev);
\draw [arrow] (method) -- (result);
\draw [arrow] (result) |- (conc);
\draw [arrow] (litrev) -- (method);
\draw [arrow] (litrev) |- (conc);

\end{tikzpicture}
\end{center}
\caption{Flow chart explaining something} \label{fig:flo}
\end{figure}

\end{document}

Current Output:

Ideal Output:

Excuse my bad MS Paint skills, but I want it to look like this:

I would like both of the yellow background planes to be symmetrical and identical in terms of dimensions.

Answer 1

Very easy with the fit and backgrounds libraries. The aux coordinate is introduced to have symmetric backgrounds.

\documentclass[a4paper, 11pt]{report}
\usepackage[toc,page]{appendix}
\usepackage{pgfgantt}
\usepackage{geometry}

\geometry{
 a4paper,
 total={170mm,257mm},
 left=20mm,
 top=20mm,
 }

\usepackage{float}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric, arrows, shadows}
\usetikzlibrary{fit,backgrounds} % <-added
\begin{document}

\tikzset{rect/.style={rectangle, rounded corners, minimum width=4.5cm, minimum
height=1cm,text centered, draw=black, fill=blue!10, drop shadow},
arrow/.style={thick,->,>=stealth}}

\begin{figure}[H]
\begin{center}
\begin{tikzpicture}[node distance=2cm]

\node (intro) [rect] {Introduction};
\node (method) [rect, below right = 0.75cm and -1.1cm  of intro,
label={[font=\small\sffamily,name=label1]above right:{Quantitative analysis}}] {Methodology \& Model};
\node (litrev) [rect, below left = 0.75cm and -1.1cm  of intro,
label={[font=\small\sffamily,name=label2]above left:{Theoretical analysis}}] {Literature Review};
\node (result) [rect, below right = 2.50cm and -1.1cm  of intro] {Results};
\node (conc) [rect, below= 4.250cm of intro] {Conclusion};

\draw [arrow] (intro) -| (method);
\draw [arrow] (intro) -| (litrev);
\draw [arrow] (method) -- (result);
\draw [arrow] (result) |- (conc);
\draw [arrow] (litrev) -- (method);
\draw [arrow] (litrev) |- (conc);
\coordinate (aux) at (result.south -| litrev);

\begin{scope}[on background layer]
\node[draw,dashed,gray,rounded corners,fill=yellow!50,fit=(method) (result) (label1)]{};
\node[draw,dashed,gray,rounded corners,fill=yellow!50,fit=(litrev) (label2) (aux)]{};
\end{scope}
\end{tikzpicture}
\end{center}
\caption{Flow chart explaining something} \label{fig:flo}
\end{figure}

\end{document}

EDIT: For completeness: I'd use the shadows.blur library, and it is really straightforward to adjust the boundary of the yellow background box with little tricks.

\documentclass[a4paper, 11pt]{report}
\usepackage[toc,page]{appendix}
\usepackage{pgfgantt}
\usepackage{geometry}

\geometry{
 a4paper,
 total={170mm,257mm},
 left=20mm,
 top=20mm,
 }

\usepackage{float}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric, arrows, shadows}
\usetikzlibrary{fit,backgrounds} % <-added
\usetikzlibrary{shadows.blur}
\begin{document}

\tikzset{rect/.style={rectangle, rounded corners, minimum width=4.5cm, minimum
height=1cm,text centered, draw=black, fill=blue!10,blur shadow},
arrow/.style={thick,->,>=stealth}}

\begin{figure}[H]
\begin{center}
\begin{tikzpicture}[node distance=2cm]

\node (intro) [rect] {Introduction};
\node (method) [rect, below right = 0.75cm and -1.1cm  of intro,
label={[font=\small\sffamily,name=label1,xshift=-3mm]above right:{Quantitative analysis}}] {Methodology \& Model};
\node (litrev) [rect, below left = 0.75cm and -1.1cm  of intro,
label={[font=\small\sffamily,name=label2,xshift=3mm]above left:{Theoretical analysis}}] {Literature Review};
\node (result) [rect, below right = 2.50cm and -1.1cm  of intro] {Results};
\node (conc) [rect, below= 4.250cm of intro] {Conclusion};

\draw [arrow] (intro) -| (method);
\draw [arrow] (intro) -| (litrev);
\draw [arrow] (method) -- (result);
\draw [arrow] (result) |- (conc);
\draw [arrow] (litrev) -- (method);
\draw [arrow] (litrev) |- (conc);
\coordinate (aux) at (result.south -| litrev);

\begin{scope}[on background layer]
\tikzset{myfit/.style={draw,dashed,gray,rounded corners,fill=yellow!50,
inner sep=10pt}}
\node[myfit,fit=(method) (result) (label1.east)]{};
\node[myfit,fit=(litrev) (aux) (label2.west)]{};
\end{scope}
\end{tikzpicture}
\end{center}
\caption{Flow chart explaining something} \label{fig:flo}
\end{figure}
\end{document}