# SIERA Definition

SIERA stands for Skill-Interactive Earned Run Average. As baseball statistics have developed, it has become clear that traditional ERA does not do a great job of identifying the true skill level of a pitcher. Therefore, a variety of other options have emerged and SIERA has come to be respected as one of the most valuable.

# The Concept of SIERA

The concept of SIERA is fundamentally the same as any other ERA alternative in that it aims to properly reward the pitcher for what he does well and penalize the pitcher for what he does poorly. For simplicity, SIERA is scaled to mirror ERA, so fans can quickly recognize a good SIERA score just by thinking about it in terms of ERA numbers. Under 3? Excellent. Over 5? Not so much.

Traditional ERA treats all runs the same, regardless of how they come about (unless they are the direct result of an error charged to a fielder). That is unfair to the pitcher, as a pitcher working with a quality defense is going to have a much better chance to prevent runs than a pitcher backed by poor defenders.

The performance of the pitcher is only one component of run prevention, so an ideal pitching stat is one that strips away those things over which a pitcher has no control. A pitcher should not be punished for playing in front of a bad defense, or for pitching in a hitter-friendly park.

# Why is SIERA Important?

SIERA is an important statistic because it provides a better picture of pitcher performance than traditional ERA.

If you were to look at ERA alone, you may overlook a pitcher who is actually performing quite well but is not getting the deserved results from an ERA perspective, for one reason or another.

For example, let’s look at Yankee’s starter Masahiro Tanaka’s performance in 2017. From an ERA perspective, you wouldn’t be too impressed, as he finished the year with a 4.74 mark. SIERA painted a much better picture, however, as his SIERA score for the season was 3.52. In other words, Tanaka was performing at a higher level than his ERA indicated, and SIERA was able to demonstrate that fact.

As a form of proof that SIERA was onto something in the 2017 season, Tanaka’s ERA dropped nearly a full run in 2018, down to 3.75. He performed very similarly in those two seasons from a strikeout and walk perspective, however fewer of his fly balls allowed turned into home runs in 2018 (something that is not typically considered to be within the pitcher’s control).

By looking at ERA, you might think Tanaka had a ‘bad’ 2017 and a ‘better’ 2018. In reality, he performed at a pretty steady level all along, and the variance of the game accounted for his vastly different ERA totals.

# How is SIERA Calculated?

The calculation for SIERA is complicated. Really complicated. In fact, to give you some idea of how complex the math is for this statistic, FanGraphs offers a five-part article to bring you up to speed.

If you’d like to dive way down deep into the nitty-gritty on this statistic, those five articles will provide what you desire. Otherwise, it’s easy enough to simply look up SIERA scores for any player in the big leagues. You can point your browser toward the FanGraphs leaderboards for pitchers, and you will have access to SIERA under the ‘Advanced’ statistics tab.

While we aren’t going to go deep into the math here, we can highlight some of the underlying numbers used within the SIERA calculation.

- Strikeouts: It’s no surprise that this stat is key in the SIERA calculation, as a strikeout is the best possible outcome for a pitcher in any given at-bat.
- Walks: On the other side of the coin, walks are a big problem for pitchers, as they turn into baserunners 100% of the time.
- BABIP: While BABIP tends to normalize toward league average over time, there are pitcher profiles which tend to live above or below the typical BABIP mark. SIERA takes these profiles (like ground ball and flyball pitchers) into account.
- Park factors: SIERA, unlike ERA, is adjusted to reflect the realities of a pitcher’s environment.

# What is a Good SIERA?

To make it easier to use, SIERA has been scaled to match up nicely with ERA. That’s helpful for baseball fans because you don’t need to relearn which numbers are good performances and which are not. If a number would be considered good from an ERA perspective, it will be a good SIERA score, as well.

Anything under 3 is excellent, a score in the low- to mid-3s is a good performance, and scores above 4 are typically below average.

The chart below includes the 10-best SIERA scores among qualified pitchers for the 2018 season:

Player |
SIERA |

Justin Verlander | 2.63 |

Max Scherzer | 2.71 |

Jacob deGrom | 2.78 |

Patrick Corbin | 2.91 |

Gerrit Cole | 2.91 |

Carlos Carrasco | 3.03 |

Trevor Bauer | 3.21 |

Corey Kluber | 3.23 |

Luis Severino | 3.26 |

Blake Snell | 3.30 |

# What are the Problems with SIERA?

SIERA is a strong statistic, and a good choice as a place to start when evaluating pitchers. There aren’t necessarily any ‘problems’ with the stat, but it is worth noting that no one statistic should be used as the final word on player performance.

For instance, looking at SIERA would be a good way to start identifying the best pitchers in the league for a given season, but you should take in other statistics as well, such as FIP and xFIP. By viewing a few different stats, you’ll get a better overall picture of the player’s performance, since those various statistics will use different inputs and will value those inputs in their own way. If a player shows up near the top of the league in a few different metrics, it’s certain that the player is one of the best in the game.