Is data augmentation changing the train/test sets distribution? I am thinking a simple question which just came to my mind.
There are many people using data augmentation on their image data to train deep CNN.
When I learn from Andrew Ng's DL courses, he mentioned that to train a better model, you generally need to have your train/test data come from the same distribution. Like, if you train you CNN with a lot of images of cars and human, it's (usually) not a good idea to use it to classify a cat or a dog.
After the augmentation, my training data is kind of boosted, then can I still say my train/test data set comes from the same distribution? Am I violating what he said? Why? Or, why not?
I would argue that augmentation does not alter your train/test sets distribution.

I say this because I consider data augmentation to be part of your training pipeline and not part of your train set.

The pipeline below is one that I use often when someone asks about data augmentation. At the moment the data is split into train/test(/validation) sets, each set should have the same, or at least very similar distributions. This is the moment you need to worry about by picking a good splitting strategy.

Then, whatever you do with the train set, including data augmentation is part of your training pipeline. You will do data augmentation, like mentioned by others, do improve you model robustness. The validation and test set, do not need to be passed through your augmentation process.

          +-&gt; training set ---&gt; data augmentation --+
          |                                         |
          |                                         +-&gt; model training --+
          |                                         |                    |
all data -+-&gt; validation set -----------------------+                    |
          |                                                              +-&gt; model testing
          |                                                              |
          |                                                              |
          +-&gt; test set --------------------------------------------------+