This is a variation on smoneck's answer which implements the east-growing tree in forest
.
I begin by demonstrating forest
's ability to space the children automatically even without additional hints.
Initial conversion
forest
uses the bracket syntax which simplifies the definition of the tree somewhat:
\documentclass[tikz,12pt]{standalone}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[english, ngerman]{babel}
\usepackage{forest}
\usetikzlibrary{positioning}
\begin{document}
\begin{forest}
for tree={
draw,
}
[Gesamtdaten, name=Root
[{$RR=0$}, name=Root-0
]
[{$RR\neq 0$}, name=Root-1
[{$0<RR\leq 0.25$}, name=Root-1-1
]
[{$0.25<RR\leq 0.65$}, name=Root-1-2
]
[{$0.65 < RR \leq 1$}, name=Root-1-3
]
[{$1 < RR \leq 1.2$}, name=Root-1-4
]
[{$1.2 < RR \leq 1.35$}, name=Root-1-5
]
[{$1.35 < RR \leq 1.5$}, name=Root-1-6
]
[{$1.5 < RR \leq 1.7$}, name=Root-1-7
]
[{$1.7 < RR \leq 1.95$}, name=Root-1-8
]
[{$1.95 < RR \leq 2.4$}, name=Root-1-9
]
[{$2.4 < RR \leq 3$}, name=Root-1-10
]
]
]
\begin{scope}[every node/.style={right}]
\path (Root-0 -| Root-1) ++(2cm,0) node[color=red] {SVM 1};
\path (Root-1-1 -| Root-1-10) ++(2cm,0) node [color=red] {SVM 2};
\end{scope}
\end{forest}
\end{document}

This spaces the nodes but unfortunately does not guarantee that the connecting lines will not cross nodes, especially when the nodes' borders are drawn.
Tweaking the appearance
We add to l
to increase the distance between levels, thicken the lines drawn throughout, and use -|
type line construction to draw the connecting lines.
\documentclass[tikz,12pt]{standalone}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[english, ngerman]{babel}
\usepackage{forest}
\usetikzlibrary{positioning}
\begin{document}
\begin{forest}
for tree={
draw,
thick,
l sep=40pt,
edge path={
\noexpand\path[thick, -, \forestoption{edge}]
(!u.parent anchor) -- +(0,-25pt) -|
(.child anchor)\forestoption{edge label};
},
}
[Gesamtdaten, name=Root
[{$RR=0$}, name=Root-0
]
[{$RR\neq 0$}, name=Root-1
[{$0<RR\leq 0.25$}, name=Root-1-1
]
[{$0.25<RR\leq 0.65$}, name=Root-1-2
]
[{$0.65 < RR \leq 1$}, name=Root-1-3
]
[{$1 < RR \leq 1.2$}, name=Root-1-4
]
[{$1.2 < RR \leq 1.35$}, name=Root-1-5
]
[{$1.35 < RR \leq 1.5$}, name=Root-1-6
]
[{$1.5 < RR \leq 1.7$}, name=Root-1-7
]
[{$1.7 < RR \leq 1.95$}, name=Root-1-8
]
[{$1.95 < RR \leq 2.4$}, name=Root-1-9
]
[{$2.4 < RR \leq 3$}, name=Root-1-10
]
]
]
\begin{scope}[every node/.style={right}]
\path (Root-0 -| Root-1) ++(2cm,0) node[color=red] {SVM 1};
\path (Root-1-1 -| Root-1-10) ++(2cm,0) node [color=red] {SVM 2};
\end{scope}
\end{forest}
\end{document}

Not only is this much too large for landscape A4, it also looks a bit squashed given the length of the final line compared with the height of the tree. So it seems worth seeing what the growth direction suggested by smoneck might do for us.
The final tree
\documentclass[tikz,12pt]{standalone}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[english, ngerman]{babel}
\usepackage{forest}
\usetikzlibrary{positioning}
\begin{document}
\begin{forest}
for tree={
draw,
thick,
l sep=40pt,
edge path={
\noexpand\path[thick, -, \forestoption{edge}]
(!u.parent anchor) -- +(20pt,0) |-
(.child anchor)\forestoption{edge label};
},
grow'=east,
child anchor=west,
parent anchor=east,
anchor=west,
}
[Gesamtdaten, name=Root
[{$RR=0$}, name=Root-0
]
[{$RR\neq 0$}, name=Root-1
[{$0<RR\leq 0.25$}, name=Root-1-1
]
[{$0.25<RR\leq 0.65$}, name=Root-1-2
]
[{$0.65 < RR \leq 1$}, name=Root-1-3
]
[{$1 < RR \leq 1.2$}, name=Root-1-4
]
[{$1.2 < RR \leq 1.35$}, name=Root-1-5
]
[{$1.35 < RR \leq 1.5$}, name=Root-1-6
]
[{$1.5 < RR \leq 1.7$}, name=Root-1-7
]
[{$1.7 < RR \leq 1.95$}, name=Root-1-8
]
[{$1.95 < RR \leq 2.4$}, name=Root-1-9
]
[{$2.4 < RR \leq 3$}, name=Root-1-10
]
]
]
\begin{scope}[every node/.style={red}]
\node [above=5pt of Root-0] {SVM 1};
\node [above=5pt of Root-1-1] {SVM 2};
\end{scope}
\end{forest}
\end{document}

The result is 107x97mm which will easily fit A4.
\tikzexternalize
and now I can compile. Before that, the error message was! Package tikz Error: Sorry, the system call 'pdflatex -halt-on-error -interact ion=batchmode -jobname "temp-figure0"
etc.sibling distance
of the second level to, for instance, 3.5cm? That is enough for the nodes not to collapse, but your figure becomes too large for the A4 format with a 12pt font: is that a problem?