Let's suppose I have a dataset which has a categorical variable and the problem I am solving is a classification one.
This categorical variable var has ['A','B','C'] as the possible set of data.
What happens to a decision tree if a new category 'D' is seen only in the validation data set (meaning: absolutely new data)? Supposing the variable var is a feature used in the tree.
With the decision tree:
Does it give an error? The decision tree stops the path and returns the not-final-node's probability?
With the Logistic Regression:
The dummy variables are zero for all the categories (I suppose), and then the model runs normally?