I have, and I still have no clue. Not going to spend time worrying about it, either. One of my top earning pieces is a short story; one is about copyright and how to take down violators online. There is very little in between, and I earned, what, forty cents last month NOT writing, but have had <$2 months writing consistently. Want to make money writing? Get someone to pay you for doing it. That's the only winning strategy I know. It's a good one, too - it let me retire in my 50s. :D