SIERA — Player Analysis
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Stat of the Week
by Peter Liubicich
SIERA — Player Analysis
This post was written by Peter Liubicich on October 25, 2012
Posted Under: Stat of the Week

We are looking at SIERA this week which is a Defensive Independent Pitching Statistic (DIPS) and measures a pitcher’s skill in a season by looking at strike outs, walks and all batted balls (ground balls, flyballs and pop ups).  This makes SIERA different from other DIPS such as FIP and xFIP as these stats consider home runs to be the only type of batted ball a pitcher truly has any control over, and stays away from ground balls, flyballs and pop ups because these can be affected by the other players lack of defensive ability, umpires and luck.

SIERA, however, is considered to be a better predictor of a pitcher’s future ERA than xFIP because it takes so many variables into consideration and a pitcher’s home run rate can fluctuate from year to year, but their ground ball and fly ball rate shouldn’t vary as much unless they completely overhaul their pitching style.

To remind you of the formula for SIERA and how it accounts for strike outs, walks, ground balls, flyballs, pop ups and plate appearances here it is courtesy 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)

Tonight’s matchup in Game 2 of the World Series pits Madison Bumgarner of the Giants against Doug Fister of the Tigers.  Both have served supporting roles in their team’s rotation (Bumgarner behind Matt Cain and Fister behind Justin Verlander), and have been very effective.

Madison Bumgarner Doug Fister
Plate Appearances 849 673
Strikeouts 191 137
Walks 49 37
Ground Balls 282 247
Flyballs 196 129
Pop Ups 35 28
SIERA 3.39 3.43

Both pitchers were ground ball pitchers and had very good strikeout to walk ratio, which gave them impressive stats in terms of SIERA.  As you can see in the equation, the coefficient for parts involving strikeouts is higher than the coefficient for equations involving walks, making having more strikeouts more favorable.  Ground balls are also favorable in the eyes of this equation since the more ground balls a pitcher induces, means less fly balls and pop ups allowed which makes the GB-FB-PU parts of the equation even higher and gets subtracted from the total SIERA number making it even lower.

One limitation in SIERA is that it does not include Line Drives, which we have explored previously, and its effect on a pitcher’s ERA in a season since they are the batted ball that is the highest producing for hitters, but may be too difficult to incorporate since it is up to the statistician or box scorer to determine if a ball was a line drive or fly ball, and may be easier to lump them all in to fly balls.

SIERA is a good indicator of things to come in future seasons, which means both Bumgarner and Fister should be valuable assets in their team’s rotation once again in 2012 (barring injuries), but it does not predict a single game as seen by last night’s game where Barry Zito, with a 2012 SIERA of 4.89, outpitched Justin Verlander, with a 2012 SIERA of 3.26.

In the future it would be interesting to look at a pitcher’s SIERA over 5-7 seasons and see how well a pitcher’s SIERA in a previous season was able to tell what their SIERA the next season would be and compare those numbers with xFIP.

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