Stat of the Week
by Peter Liubicich
This post was written by Peter Liubicich on October 22, 2012
Posted Under: Stat of the Week

Skill-Interactive ERA, more commonly known as SIERA, is another Defensive Independent Pitching Statistic that is in the family of FIP and xFIP where it is an ERA estimator and tries to better define a pitcher’s skill in a give season.  Just like FIP and xFIP, SIERA is a weighted formula where different plays that a pitcher affects in a game are given a different coefficient:

Courtsey of Baseball Prospectus:

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)

where the +/- term is a negative sign when (GB-FB-PU)/PA is positive and vice versa.

The statistics evaluated in SIERA are Strikeouts, Plate Appearances, Walks, Ground Balls, Flyballs, Pop Ups.

SIERA is different from FIP and xFIP as those two pitching statistics want to focus solely on plays that pitchers can control, Strikeouts, Walks and Home Runs, while SIERA instead includes balls that they allow in play, “but attempts  to explain why certain pitchers are more successful at limiting hits and preventing runs” according to FanGraphs.

As you can see in that formula, the statistics that mainly affect SIERA are strikeouts, walks and ground balls (since we are trying to see how many ground balls are allowed per plate appearance by a pitcher when using GB-FB-PU), and according to Baseball Prospectus, “SIERA accounts for how run prevention improves as ground ball rate increases and declines as more whiffs are accrued, while grounders are of more materiality for those who allow a surplus of runners.”

What this means is that SIERA shows that pitchers that produce more ground balls are more effective at preventing runs as although ground balls will go for hits more often that fly balls, it is easier for the defense to turn ground balls into outs or turning double plays when runners are allowed on base.

On Thursday, we’ll look at the SIERA for different ground ball and strikeout pitchers in the 2012 season and compare their numbers to their actual ERAs.

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Reader Comments

I think you have finally written something that is way over my head- I knew the day would come, and now it has!

Written By Johanna Wagner on October 23rd, 2012 @ 4:36 pm


  1. SIERA — Player Analysis  on October 25th, 2012 @ 3:00 pm

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