The Marcels are the brainchild of Tom Tango. I will let him describe his system:
The Marcel the Monkey Forecasting System (or the Marcels for short) is the most advanced forecasting system ever conceived.
Actually, it is the most basic forecasting system you can have, that uses as little intelligence as possible. So, that’s the allusion to the monkey. It uses 3 years of MLB data, with the most recent data weighted heavier. It regresses towards the mean. And it has an age factor.
The reliability column tells you how reliable the forecast is. (The higher the better).
How accurate are they, you may ask? They hold up pretty darn well. If you want to look at all hitters and pitchers in baseball on your own, you can go here to download the zip file that contains the separate files. Below are the Marcels for the Rays’ batters sorted by wOBA; I will do the pitchers later tonight.