#ordinal-encoding

Question 1: Single choice

You fit scikit-learn's OrdinalEncoder on the categorical columns of the training data and then call transform on the validation data, which raises an error. What is the most likely cause?

The validation data has categories never seen in training1
The validation data has fewer rows than the training data had2
The categories have no natural order, which ordinal encoding requires3
The encoder has to be fit on the validation data as well, separately4

Timed test