I am writing a decision tree trained with the ID3 algorithm from scratch. I wanted to be able to train on and classify continuous data, so I implemented k-means clustering and reduced the range of values of any input training or predicting data.

However, I ran into the problem where an attribute value that was not encountered during training, in a deep node somewhere in the tree, but that exists further up was encountered. All data points with this specific attribute value probably ended up on a different branch of the tree.

So to 'solve' this, whenever an unknown attribute value is encountered for a node, I sent it down a random existing branch.

I get 95.45% accuracy using randomly split 85%-15% training-test data with iris.

Is this an acceptable approach to take or have I gotten something wrong here?

Here's the code: https://github.com/jamalmoir/ml_components/blob/master/ml_components/models/decision_tree.py



1 Answer 1


Do your leaf nodes return a probability distribution rather than a single class value e.g., the majority class of training instances that arrived at the node? If so, a better approach would be to send the test instance down every child path and average the probability distributions that are returned by the leaves.

Your good score of 95.45% on iris data suggests to me that it's not a very important detail in this example. Try some more challenging datasets and see if it makes a difference.

  • $\begingroup$ My leaf nodes just return a class label... Maybe I could look into probability distributions then. Thanks for the suggestion! I'll take a look at more challenging datasets. Thanks $\endgroup$
    – Jamal Moir
    Feb 23, 2017 at 4:53
  • $\begingroup$ It's a good idea to be returning distributions from your decision trees, because if you want to make an ensemble of decision trees, averaging the distributions will give you a more accurate classification than simply voting. $\endgroup$ Feb 23, 2017 at 20:18
  • $\begingroup$ I see, that's a good point. Thank you. I will definitely work on building probability distributions into my decision trees. $\endgroup$
    – Jamal Moir
    Feb 24, 2017 at 4:52

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