2009 Yankees Through the Lens of SIERA

Written by Greg Fertel | 12 February 2010

Baseball Prospectus has been introducing their new defense-independent pitching stat dubbed SIERA(Skill-Interactive Earned Run Average). They have had five introductory posts explaining everything about the statistic:

Here's an excerpt from part 1, detailing what SIERA intends to do:

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.

For more information on SIERA, there is a good discussion going on about it at The Book Blog.

Here is what the final formula is to calculate the new metric:

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 +/- is as before such that it is a negative sign when (GB-FB-PU)/PA is positive and vice versa.

Now, here's how the 2009 Yankees look:

The top four guys were the Yankees best relievers in 2009, so I'm not surprised to see them so high. David Robertson's number is extremely impressive and I'm curious to see how he ranks among all relief pitchers. He should be an asset in the bullpen over the next few seasons.

Phil Hughes and Alfredo Aceves have numbers right in line with their 2009 ERAs, which is nice to see. Using a one year sample for relievers isn't particularly effective, but it does suggest that their strong seasons were not flukes.

The biggest surprise to me was how well Sergio Mitre fared. xFIP really likes Mitre too, and I believe that SIERA and xFIP are the most predictive pitching metrics out there. Yankee fans hated Mitre last season, but these numbers suggest he was extremely unlucky and could rebound in 2010. Seeing this makes me think holding onto Mitre was the right decision.

Joba Chamberlain comes in with a solid 4.41 SIERA. There is no doubt he was frustrating to watch in 2009, but he wasn't as bad in the rotation as he has been made out to be. He was only a bit below average with FIP and ERA while SIERA suggests that he was possibly above average.

I like the methodology behind this new metric, so you can expect to see me using it here throughout the season. It is similar to tERA in that it uses batted ball data, but it seems to have higher correlation scores. You can definitely expect to see me using it alongside FIP numbers as we go through the 2010 season.

Edit: I forgot to include Vazquez in the above chart, but his 2009 SIERA came out to 2.68, wow.