Baseball Prospectus:  Introducing SIERA

Matt Swartz and Eric Seidman introduce another way of measuring pitcher effectiveness:

[W]e have invented a new statistic, Skill-Interactive Earned Run Average (SIERA), which corrects the problems with old estimators while adding a few more realistic assumptions. This was done first by un-foiling all of the individual components in QERA while making an adjustment for the issue with the ground-ball denominator issue, and testing to see which interactions and squared terms were relevant by using multiple linear regression analysis. Essentially, we changed the GB/BIP to (GB-FB-PU)/PA and evaluated all of the terms in the exponential regression, removing those with insignificant p-values; while the QERA formula only shows three variables, un-foiling the formula reveals several more. We identified two terms that were not useful: the squared term of walks, and the interaction between walk and strikeout rate. The squared terms on strikeout and ground-ball rates were both significant, and we also found important interactions between walks and grounders and between whiffs and grounders that have strong effects on run scoring.

As a result, SIERA accomplishes the following:

1.  Allows for the fact that a high ground-ball rate is more useful to pitchers who walk more batters, due to the potential that double plays wipe away runners.
2.  Allows for the fact that a low fly-ball rate (and therefore, a low HR rate) is less useful to pitchers who strike out a lot of batters (e.g. Johan Santana’s FIP tends to be higher than his ERA because the former treats all HR the same, even though Santana’s skill set portends this bombs allowed will usually be solo shots).
3.  Allows for the fact that adding strikeouts is more useful when you don’t strike out many guys to begin with, since more runners get stranded.
4.  Allows for the fact that adding ground balls is more useful when you already allow a lot of ground balls because there are frequently runners on first.
5.  Corrects for the fact that QERA used GB/BIP instead of GB/PA (e.g. Joel Pineiro is all contact, so increasing his ground-ball rate means more ground balls than if Oliver Perez had done it, given he’s not a high contact guy).
6.  Corrects for the fact that FIP and xFIP use IP as a denominator which means that luck on balls in play changes one’s FIP.

Part 2 of the introduction can be found here.