Numerous alternatives to ERA have sprung up the fast few yeara - FIP, xFIP, QERA, etc. The theory is that these component ERAs are truer measures of a pitcher's run prevention skill than ERA because they focus on events that he can control - strikeouts, walks, home runs, fly balls, ground balls. Pitchers have less control over how many hits they allow because they share the responsibility of hits with fielders. Thus, hits are not included in any of these formulas.
The ERA estimators are also independent of sequencing of events. For example, if a pitcher allows a leadoff home run followed by two singles, he will only allow one run. However, if he allows two singles and then allows the home run, he'll be charged with three runs. Theoretically, he pitches equally as poorly in both instances but the number of runs that score is different simply because of sequencing. How much control individual pitchers have over sequencing is still open to debate but there is no evidence that most pitchers can consistently control it to a great extent.
These new estimators have proven to be somewhat better than current ERA at predicting future ERA. Eric Seidman and Matt Swartz of BaseballProspectus.com recently developed Skill Interactive Earned Run Average (SIERA) which looks to be a further improvement over FIP, XFIP and QERA. The math behind SIERA is very complicated and won't be presented here but you can think of it as the ERA that a pitcher should have had based on his rate of strikeouts, walks, ground balls, fly balls and pop ups.
One of the benefits of SIERA is that it considers the interaction between events whereas other measures look at events separately. For example, SIERA considers that a high ground ball rate helps a pitcher who walks a lot of batters more than it helps one who doesn't. This is because walks and ground balls together can potentially create double plays. A pitcher with a high ground ball rate and high walk rate may look better on SIERA than he does on QERA.
Similarly, a high fly ball rate (and an accompanying high home run rate) is less damaging to pitchers like Johan Santana who strike out a lot of batters and walk few batters. This is because they allow few baserunners and their home runs tend to be solo shots. Santana tends to have a FIP which exceeds his ERA because FIP counts all home runs equally for all pitchers. A pitcher like this will tend to have a lower SIERA than FIP.
Looking at Tigers pitchers in 2009, Justin Verlander had a SIERA of 2.80 (his FIP was 2.84) which is much lower than his 3.45 ERA. The reason for this discrepancy is that he tended to allow runs in bunches last year which inflated his ERA. In theory, you would expect him to have an ERA closer to 2.80 than 3.45 next year if his K, BB and batted ball rates are similar. Verlander had the top SIERA in the American League last year.
Max Scherzer also did better on SIERA (3.54) than he did on ERA last year (4.12) which is one reason to be optimistic about his future. Interestingly, the pitcher they traded for him - Edwin Jackson - went in the other direction posting a 4.21 SIERA and 3.62 ERA.
On the downside, Rick Porcello had a 4.41 SIERA compared to a 3.96 ERA. His FIP was even higher at 4.88. The reason for the discrepancy in his SIERA and FIP was his high ground ball rate, an event not included in FIP.
The data contained in this article were provided by Cubs Stats.