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Decision Tree Diagrams [Decision Trees
Posted on March 21, 2016 @ 06:20:00 AM by Paul Meagher

I've been reading quite a bit about decision making lately and one common recommendation for making better decisions is to diagram your decision using decision trees.

In Reid Hastie & Robyn M. Dawes textbook Rational Choice in an Uncertain World: The Psychology of Judgement and Decision Making (2nd Edition, 2010) they use the convention that decision alternatives are indicated by a small square and event alternatives are indicated by a small circle. I used the online diagramming tool draw.io to create the skeleton of a basic decision tree diagram:

In a fully specified decision tree diagram I would also have labels along each horizontal branch, probabilities along each event branch, and consequence values at the terminal end of each event branch (see this previous blog for an example). I won't be going into such details in this blog because I want to focus on the first step in diagramming a decision tree; namely, drawing the skeletal structure of a decision tree.

If you want to diagram decisions it is useful to have a tool that makes it easy to do so. A piece a paper is a good option, but I wanted to use a piece of software to create decision tree diagrams exactly as they appear in the textbook mentioned above. The draw.io tool fits these requirements. In a previous blog I demonstrated a more complex approach to creating decision trees. In this blog I wanted to find a more accessible approach that anyone could use without installing a bunch of software.

Decision trees have lots of symmetrical branches and drawing them was the main challenge I had in making decision tree skeletons. Duplicating and dragging horizontal or vertical lines were the main actions I ended up using to create the symmetrical decision tree diagram above. The left column in the figure above shows the "General" and the "Misc" symbol libraries that I used to create the square, circle, and line shapes. By default they are available to use when you access the draw.io online application and you just need to click the library name to open it and access the diagramming shapes you want to use.

It would be possible to print off a decision tree template like this so you can write labels and numbers onto it. It would be a ready-made template for a common form of decision problem, namely, a decision with 2 possible choices and 2 possible event outcomes that will determine the expected consequences for each choice.

In this blog my goal was to suggest some software you can use to create decision tree diagrams (draw.io) and how you can go about creating nice symmetrical tree diagrams with it. There are probably more efficient techniques to use than what I'm suggesting as I just started playing around with draw.io for this purpose and the duplicate and drag approach was the first approach that worked ok. We can use decision tree diagrams to explore a wide range issues in decision making and in future blogs we'll explore some of these issues.

Here are some useful tips that will get you up to speed quickly using draw.io.

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