We removed the Easy/Normal/Hard buttons from our app, partly because they're often confusing
to new users, and because we don't feel like they add a lot to the learning experience. In our view,
collecting that data doesn't add enough to the flashcard optimization process to really be worth
the added user interface clutter.
Down the road, we're looking into other ways of monitoring how challenging a given word is, so
that we can fine tune our algorithm to better fit language learning needs without sacrificing a
clean and easy user experience.