Hi, Miguel:
Each campaign's target is a binary variable, i.e., Y=1 if customer click and Y=0 if customer didn't click on the web link.
I actually built a decision tree with nominal target variable exactly by doing what you suggested. However, my concern is that not all customers were exposed to all my 4 campaigns.
In other words, it didn't really make sense to say that customer John prefered Campaign A over Campaign B, because John was only exposed to Campaign A, but not Campaign B.
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