Why SIERA Might Actually Make Sense

When Baseball Prospectus introduced Skill-Interactive Earned Run Average (SIERA), it's version of a defense independent pitching measurement, I yawned. It struck me as a redundant and yet another minor refinement to DIPS, FIP or xFIP. We already have this measurement....why do we need this new one? It just seemed to be an attempt to come up with a new proprietary stat that was not necessary.

I don't know if I missed the back story or if this in just hte first time someone has explained SIERA like this but Matt Swartz lays out why SIERA is different (and perhaps better) than FIP and it's incarnations:


It sometimes seems as if the main reason people are wary of Defense Independent Pitching Statistics as a way to measure pitching performance is that they are reluctant to believe the theory that pitchers do not control the hit rate on balls in play (BABIP). It does not make intuitive sense, and it isn't even entirely true...

When I say that these defense independent pitching statistics "explain" BABIP skill, I mean that you can figure how good pitchers are at preventing hits on balls in play by knowing how good they are at these skills. Specifically, pitchers who strike out a lot of hitters also induce a lot of weak contact. Randy Johnson was good at preventing hits on balls in play, and Tim Lincecum looks pretty good, too. Additionally, pitchers who walk a lot of hitters—those who miss the corners of the plate by a few inches—also have trouble with leaving the ball a few inches towards the middle of the plate, and are more likely give up more hits. Greg Maddux is an example of the opposite extreme as his impeccable control allowed him to keep the hits down. Pitchers who allow a lot of fly balls also induce a lot of pop-ups, easy outs that keep BABIP down. Both Jered Weaver and Ted Lilly are fly ball-prone, and while that hurts their home run numbers, they also induce a fair amount of popups. The ability to control the ball, miss the bat, and hit the top of the bat all correlate very highly and explain most of the pitcher's ability to control BABIP, and this is all utilized by SIERA.


This is interesting because it's the first stat (that I know of) that acknowledges a pitcher skill beyond the Three True Outcomes. I am certainly stat saavy, not a stat guru but this looks like a significant step forward in defense independent pitching assessments.

All that said, good luck finding the stat on the site. This is my first season with a BP subscription and I find the structure of the site maddening. But that's another story.

At some point, it would be interesting to see how SIERA views pitch-to-contact types like Jeremy Guthrie and Brad Bergesen.