Forest: classic decision tree

Question

I am trying to create a "classic" decision tree with forest. I have never used forest before and struggle with a couple of things. Unfortunately, I did not find the documentation an easy read. I found an answer that broad me a bit along the way (https://tex.stackexchange.com/a/251452/36836). I have adapted and changed it a bit. However, I would like to make the following further changes (partly) illustrated in the diagram below.

1. Move the node labels (n_0, n_1, etc.) above the nodes (boxes/circles).
2. Let the branches start at the east of the nodes (so that they don't interfere with the node labels.
3. Make the nodes (boxes/circles) smaller (since they have no content anymore).
4. Remove the triangles from the end and put the outcomes, o_1, o_2, etc, at directly after the edge (where the triangles were).
5. Possibly, change the ordering so that the numbering starts from above rather than below, e.g. n_1 should be the node on top above n_2 and n_3.
6. Move all settings into the preamble so that they are applied to all trees (maybe naming it "decision tree").

\documentclass{article}

\usepackage{etex,forest}

\usetikzlibrary{shapes,snakes}

\tikzset{
    decision/.style={rectangle, minimum height=10pt, minimum width=10pt, draw=black, fill=white, thick, inner sep=0pt},
    chance/.style={circle, minimum width=10pt, draw=black, fill=white, thick, inner sep=0pt},
    leaf-chance/.style={isosceles triangle, minimum width=10pt, draw=black, thick, fill=white, inner sep=0pt, shape border rotate=180, outer sep=-\pgflinewidth}
  }

\forestset{
  declare toks={optimality}{},
  sub optimal/.style={
    optimality={
      node [midway, sloped]
    }
  },
  declare toks={edge label below}{},
}

\begin{document}
\begin{forest}
   my label/.style={
     edge label={node[midway,above,font=\scriptsize,text width=1.5cm,align=left]{#1}}
   },
    for tree={
      grow=0,
      font=\scriptsize,
      child anchor=west,
      anchor=west,
      text ragged,
      inner sep=1mm,
      edge={thick},
      l sep+=15mm,
      s sep+=5mm,
      if n children=0{
        before typesetting nodes={
          label/.wrap pgfmath arg={right:#1}{content()},
          content={},
          leaf-chance,
        },
      }{},
      edge path={
        \noexpand\path[\forestoption{edge}]
        (!u.parent anchor) -- ([xshift=-15mm].child anchor)\forestove{optimality} --  (.child anchor)\forestoption{edge label below}\forestoption{edge label};
  },
    }
    [$n_0$,decision
      [$n_1$,decision,my label={$s_1$, $s_2$}
        [$o_1$,chance,my label=$s_1(n_1)$]
        [$o_2$,decision,my label=$s_2(n_1)$]
      ]
      [$n_2$,decision,my label={$s_3$, $s_4$}
        [$o_3$,decision,my label=$s_3(n_2)$]
        [$o_4$,decision,my label=$s_4(n_2)$]
      ]
      [$n_3$,decision,my label={$s_5$, $s_6$}
        [$o_5$,decision,my label=$s_5(n_3)$]
        [$o_6$,decision,my label=$s_6(n_3)$]
      ]
    ]
\end{forest}
\end{document}

Answer 1

like this?

edit: an attempt to make tree code slightly shorter

• from preamble of your mwe is relevant only package forest
• styles of nodes and edges labels are defined as for tree options
• style for labels is moved in forest environment
• to tree code are added comments for short its explanation

\documentclass[margin=3mm]{standalone}
\usepackage{forest}

\begin{document}
\begin{forest}
/tikz/every label/.append style={xshift=-1pt,inner sep=1pt,font=\footnotesize},
  for tree={
    grow'=east,
  % node style
    line width=0.4pt,
    minimum size=2pt,
    inner sep=2pt,
    if n children=0{}{draw}, % <--- added
  % anchors <------- solve basic problem 
    parent anchor=children,  
    child anchor=parent,     
  % distances of nodes
      l sep=17mm,           % <-- *l*: level distance
      s sep=2mm,            % <-- *s*: sibling distance between leaves
  % edges
    edge={draw},
    edge path'={
       (!u.parent anchor) -- ([xshift=-13mm].child anchor) -- (.child anchor)
                },
  % labels on edges
    delay={
      edge label/.wrap value={node[midway, inner sep=1pt,
                                   font=\scriptsize, above]{#1}},
           },
  }, % end for tree
  [ , label=$n_0$
    [, label=$n_3$, edge label={$s_5,s_6$}
      [$o_6$, edge label={$s_6(n_3)$}]
      [$o_5$, edge label={$s_5(n_3)$}]
    ]
    [ , label=$n_2$, edge label={$s_3,s_4$}
      [$o_4$, edge label={$s_4(n_2)$}]
      [$o_3$, edge label={$s_3(n_2)$}]
    ]
    [ , label=$n_1$, edge label={$s_1,s_2$}
      [$o_2$, edge label={$s_2(n_1)$}]
      [$o_1$, edge label={$s_1(n_1)$}]
    ]
  ]
\end{forest}
\end{document}

Answer 2

\documentclass{article}
\usepackage{forest}

\forestset{
  declare toks={elo}{},
    my label/.style={
        edge label={node[midway,above,text width=1.7cm,align=left]{#1}}
    },
    decision tree/.style={
    for tree={
        grow=east,
        minimum size=2pt,
        inner sep=2pt,
        parent anchor=east,
        child anchor=west,
        l sep+=20mm,
        s sep+=3mm,
        edge path'={(!u.parent anchor) -- ([xshift=-18mm].child anchor)\forestove{elo} -- (.child anchor)},
    }
    }
}

\begin{document}
{\small
\begin{forest} decision tree
    [,label=$n_0$,draw
      [,label=$n_3$,draw ,my label={$s_5,s_6$}
        [$o_6$, my label=$s_6(n_3)$]
        [$o_5$, my label=$s_5(n_3)$]
      ]
      [,label=$n_2$,draw, my label={$s_3,s_4$}
        [$o_4$, my label=$s_4(n_2)$]
        [$o_3$, my label=$s_3(n_2)$]
      ]
      [,label=$n_1$,draw, my label={$s_1,s_2$}
        [$o_2$, my label=$s_2(n_1)$]
        [$o_1$, my label=$s_1(n_1)$]
      ]
    ]
\end{forest}}
\end{document}