# Why is Random Forest feature importance biased towards high cadinality features?

I understand how a random forest algorithm works but could someone tell me the rationale behind Random Forest feature selection being biased towards high cardinality features?

• there are different ways to calculate feature importance and the answer depends on the implementation. anything in particular you are referring to?
– oW_
May 14 '19 at 23:02
• I am specifically talking about Random Forest variable importance. May 14 '19 at 23:17
• There are different ways to calculate feature importance in random forests - variance and permutation importance are two examples of techniques. May 15 '19 at 10:17

Random forest (as almost any other algorithm) is prone to selecting variables which can lead to a one-to-one relationship with the $$Y$$ variable.