If you had to bet your life on the accuracy of an ERA forecast made by one stat, what would it be? Mine would be xFIP, which is easily available at The Hardball Times. First of all, here's how they define it...
xFIP: Expected Fielding Independent Pitching. This is an experimental stat that adjusts FIP and "normalizes" the home run component. Research has shown that home runs allowed are pretty much a function of flyballs allowed and home park, so xFIP is based on the average number of home runs allowed per outfield fly. Theoretically, this should be a better predicter of a pitcher's future ERA.
and here's the definition of FIP (which they referenced above)...
FIP: Fielding Independent Pitching, a measure of all those things for which a pitcher is specifically responsible. The formula is (HR*13+(BB+HBP)*3-K*2)/IP, plus a league-specific factor (usually around 3.2) to round out the number to an equivalent ERA number. FIP helps you understand how well a pitcher pitched, regardless of how well his fielders fielded. FIP was invented by Tangotiger.
So what's so great about this? Its a relatively quick and dirty calculation that puts its weight on the factors that pitchers DO control (Ks, BBs, GB/FB ratio) and eliminates the factors that are basically out of their control (what happens on balls in play, whether an outfield fly ball happens to make it over the wall or not).
And what's not so great about it? Well, a few things:
1. Ideally you want to factor in the pitcher's team's expected defensive efficiency when making a forecast.
2. Pitchers actually do have a little control over what happens on balls in play. Knuckleball pitchers in particular seem to have an edge here.
3. While Outfield FB % is a better predictor of HR allowed than HR is, its not perfect...probably some weighted combination of the two would be better yet.
4. xFIP (at least as used here) is based on just one season's data. A weighted average of multiple seasons would probably be better.
5. Above all...this is an art, not a science. No single stat can show everything...for example, was the pitcher playing with an injury? Did he learn a new pitch in the offseason? Its trying to put all these factors together that make fantasy baseball fun to play!