I'm taking a class now and they have contradicted themselves, so I'm looking for some clarification.
Can you standardize your values on the ENTIRE data set or does it have to be done only on train (because of data leakage). What about creating dummy variables, inputing missing values, and hyper parameter tuning.
Originally they said all of these had to be fit on train and then transformed on test. Now they are saying it can be done on the entire data set. Can anyone help me understand which is correct?