I got the following flow chart diagram in tikz.

MWE
\documentclass{standalone}
\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usetikzlibrary{calc, shapes, arrows, positioning}
\begin{document}
% Define block styles
\tikzstyle{block} =
[
rectangle
, draw
% , fill=blue!20
, text width=12.5em
, text centered
, node distance=1.5cm
, rounded corners
, minimum height=2em
]
\tikzstyle{block2} =
[
rectangle
, draw
% , fill=blue!20
, text width=6.5em
, text centered
, node distance=1.5cm
, rounded corners
, minimum height=2em
]
\tikzstyle{line} =
[
draw
, -latex'
]
\begin{tikzpicture}[node distance = 1cm, auto]
% Place nodes
\node [block] (ZeroSlopes) {Test Slopes are equal to Zero, i.e.,\\ $\textrm{H}_{0}:\beta_{1}=\beta_{1}=\ldots=\beta_{t}=0$};
\node [block, below left = 2.0cm and 0.1cm of ZeroSlopes] (CommonSlopes) {Test Slopes are equal, i.e.,\\ $\textrm{H}_{0}:\beta_{1}=\beta_{1}=\ldots=\beta_{t}=\beta$};
\node [block, below right = 2.0cm and 0.1cm of ZeroSlopes] (ZeroSlope) {Fit Common Slope \& Test Slope is equal to Zero, i.e.,\\ $\textrm{H}_{0}:\beta=0$};
\node [block2, below left of=CommonSlopes, node distance=4.5cm] (UnequalSlopes) {Fit Unequal Slopes Model};
\node [block2, below right of=CommonSlopes, node distance=4.5cm] (CommonSlope) {Fit a Common Slope Model};
\node [block2, below left of=ZeroSlope, node distance=4.5cm] (ParallelLines) {Fit Parallel Lines Model};
\node [block2, below right of=ZeroSlope, node distance=4.5cm] (ANOVA) {Fit ANOVA Model};
% Draw edges
\path[line] let \p1=(ZeroSlopes.south), \p2=(CommonSlopes.north) in (ZeroSlopes.south) -- +(0,0.5*\y2-0.5*\y1) -| node [pos=0.3, above] {Reject} (CommonSlopes.north);
\path[line] let \p1=(ZeroSlopes.south), \p2=(ZeroSlope.north) in (ZeroSlopes.south) -- +(0,0.5*\y2-0.5*\y1) -| node [pos=0.3, above] {Retain} (ZeroSlope.north);
\path[line] let \p1=(CommonSlopes.south), \p2=(UnequalSlopes.north) in (CommonSlopes.south) -- +(0,0.5*\y2-0.5*\y1) -| node [pos=0.3, above] {Reject} (UnequalSlopes.north);
\path[line] let \p1=(CommonSlopes.south), \p2=(CommonSlope.north) in (CommonSlopes.south) -- +(0,0.5*\y2-0.5*\y1) -| node [pos=0.3, above] {Retain} (CommonSlope.north);
\path[line] let \p1=(ZeroSlope.south), \p2=(ParallelLines.north) in (ZeroSlope.south) -- +(0,0.5*\y2-0.5*\y1) -| node [pos=0.3, above] {Reject} (ParallelLines.north);
\path[line] let \p1=(ZeroSlope.south), \p2=(ANOVA.north) in (ZeroSlope.south) -- +(0,0.5*\y2-0.5*\y1) -| node [pos=0.3, above] {Retain} (ANOVA.north);
\end{tikzpicture}
\end{document}
Question
I've problem with adjusting the horizontal and vertical distances between the nodes. You can see that the nodes horizontally are far apart from each other. Is there any better better approach to make flow charts with tikz? Any help to resolve the problem will be highly appreciated. Thanks in advance.

