Curriculum learning is an interesting area of study within machine learning. However, it is often applied in a somewhat hand-wavy fashion. What does it mean for a particular training example to be harder than another one?
Sometimes, sentence length is used to indicate difficulty. But in the context of movie reviews, “meh” could be positive or negative (in a binary task), while a long tirade against a movie would very obviously be negative. Similarly, rarer sentences, phrases are often considered harder. But if someone comes up with a unique way of praising your film, that would be an easy one! Neutrality only has so many expressions.
Difficulty itself as a concept is slippery. It could be associated with human or model perception of difficulty, or could relate to a specific aspect of a task. It’s worth being careful and defining terms when setting up a curriculum learning procedure.