Should you perform feature scaling/mean normalization when predicting with a model?

Question

Should you also perform feature scaling/mean normalization when predicting with a model, that was trained and tested on with feature scaling/mean normalization?

Answer 1

Yes, testing data should follow the same preprocessing as the training data. Otherwise, testing data will have nothing comparable with what the algorithm learned, leading to (very) bad performances.

note: In Sklearn, the Pipeline class helps you to respect the fundamentals of ML modeling like data leakage and applying the same transformations to train and test set.