So far, we have talked about two different ERA estimators, FIP and xFIP. These metrics exclude the complications of a team’s defense to isolate the factors that pitchers have more control over. Because of that, FIP and xFIP are both called “Defense Independent Pitching Statistics”, or DIPS.
SIERA, or Skill Interactive Earned Run Average, is a stat that was created to account for the quality of contact on batted balls to better analyze a pitcher’s sole impact on their performance.
SIERA Explained
Think of SIERA as the wOBA of ERA estimators. Instead of excluding batted balls or weighing them all the same, SIERA attempts to evaluate the skill of a pitcher by adding the correct weights to these outcomes, which make up the majority of plate appearances.
SIERA is represented by the following equation:
SIERA = 6.145 – 16.986 (SO/PA) + 11.434 (BB/PA) – 1.858 ((GB-FB-PU)/PA) + 7.653 ((SO/PA)^2) +/- 6.664 (((GB-FB-PU)/PA)^2) + 10.130(SO/PA)((GB-FB-PU)/PA) – 5.195(BB/PA)*((GB-FB-PU)/PA)
*The +/- term is a negative sign when (GB-FB-PU)/PA is positive and vice versa
The idea behind SIERA is that a pitcher’s ability to get swings and misses, and limit walks and home runs are not the only important skills that lead to their success, or predict future performance.
While batted balls are random and complicated, certain outcomes like ground balls and weak contact are more desirable and have predictive value. SIERA incorporates how often a pitcher is achieving desirable outcomes into the ERA value. SIERA does a slightly better job of predicting future ERA than FIP and xFIP, but you shouldn’t abandon those other metrics altogether.
Unlike FIP and xFIP, SIERA is park-adjusted as well. For example, if you’re playing in a hitter’s park, you’re at more of a disadvantage than when playing in a pitcher’s park. Thus, it’s important to account for these factors that can affect your performance.
As far as how to interpret SIERA, it is scaled similarly to ERA which you’re already used to. For example, a pitcher might have an ERA of 3.85 and their SIERA may be displayed as 3.35.
Summary
The type of run estimator you use to evaluate a pitcher should depend on what skills you value most. To get the best picture it’s most useful to use and understand all three in conjunction with one another. SIERA adds more information to this conversation by accounting for batted ball outcomes and their significance on pitcher performance.
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