Before revision: Text data augmentation, i.e.  the creation of synthetic textual data from an original text, is challenging as augmentation transformations should take into account language complexity while being relevant to the target Natural Language Processing (NLP) task (e.g.  Machine Translation, Question Answering, Text Classification, etc.).
After revision: Text data augmentation, i.e. the creation of synthetic textual data from an existing text, is challenging as augmentation transformations should take into account language complexity while being relevant to the target Natural Language Processing (NLP) task (e.g. Machine Translation, Question Answering, Text Classification, etc.).