From here, we turn from batters to pitchers. A pitcher controls a few things about the outcome of an at bat, each to a variable degree:
From all of the aforementioned reasons, it is important to look beyond ERA and WHIP in evaluating a pitcher's true ability beyond the success he does or does not experience in a single season. ERA measures single-season results with all the non-neutral luck variables factored in. Using ERA to identify a pitcher's talent is like trying to identify a pieces of strawberry in a finely blended all-berry smoothie (food metaphor). We need to look beyond what a pitcher does not control and into what a pitcher does control. This is where FIP or Fielding Independent Pitching statistics (which you may know it as DIPS, or Defense Independent Pitching Statistics) comes in handy. The statistical analysis grunt work has already been done (thanks to Tom Tango) and the coefficient of determination is pretty high (approximately five times that of ERA).
FIP is a better measure of future success than ERA. FIP essentially takes the three elements a pitcher exerts the most control over (K's, BB's and HR's) and weights them in regard to their historical impact on ERA and then adds to them a league-specific factor to round out the resulting number to an equivalent ERA number. The formula for calculating FIP is 3.2+((13*HR)+(3*(BB+HBP-IBB))-(2*K))/IP. ERA tends to regress towards FIP over time. It likely will not match it (too many variables at play), but you can determine whether a player is likely to improve or regress in the future based on FIP-ERA differentials (FIP-based trading and team management is a cornerstone of fantasy baseball success).
But wait, DME, you say, didn't you just say, and I quote, "a pitcher does not control home runs; only the tendency thereof?" You are correct and this brings me to the advanced form of FIP I prefer to utilize called expected FIP or xFIP ("x" is a general term you see in front of statistics to represent some predicative value). xFIP is similar to FIP in how it incorporates the historical weight of K's, BB's and HR's on ERA, but whereas FIP uses a pitcher's in-season home run totals to calculate a pitcher's in-season FIP, xFIP uses a metrics we might call xHR, where xHR is a specific function of the league average HR/FB rate. xFIP measure's a player's future ERA under the theory of HR/FB rate regression.There is another FIP-like metric out there called tRA, which utilizes a pitcher's total batted ball profile to evaluate his luck-neutral results from a single season and predict a player's future ERA. tRA is not exactly scaled to reflect ERA, so it can be a bit confusing. I personally do not use tRA because I do not believe a pitcher controls "line drives" (at least not as classified and encapsulated in box score data).From all this explanation and ramble, we derive the following maxims. A pitcher who walks less batters, strikes out more batters and keeps the ball on the ground is likely to succeed. A pitcher who induces more contact and pitches for the 2007 White Sox is not.