Sunday, December 24, 2006

Ground ball Pitchers

ERA has traditionally been used to evaluate overall pitching performance but it is problematic because it does not separate pitching from fielding. More and more, I've been using FIP ERA (or ERA) which is calculated using only statistics over which a pitcher has control - strikeouts, bases on balls, hits batsmen and home runs. FIP is more predictive or more stable from year to year than ERA.

In my article on repeatable pitching skills, I showed that SO/IP (correlation=.77) and BB/IP (correlation=.67) are very repeatable from one year to the next. Home runs per IP is less consistent (.38) and it is also park dependent. For this reason, many analysts are now using ground ball percentage (percent of batted balls which are hit on the ground) more than home runs. Ground ball percentage is relatively consistent (.73) and is also not very dependent on ballpark. One disadvantage of ground ball rate is that it is not an actual outcome. Getting a batter to hit a ground ball is fielding independent but getting the ground ball to turn into an out is not.

The statistic QERA, developed by Nate Silver at Baseball Prospectus, uses GB% instead of home runs:

QERA= (2.69 - SO% x .34 + BB% x 3.88 - GB% x .66)^2

While QERA is more predictive of future performance than ERA (.51 versus .33), I have not found it to be more predictive than FIP (.54). Still, GB% by itself is a useful tool which helps to describe an important pitching skill.

While a ground ball does not always produce a good result for a pitcher, it’s much less likely to produce a bad result than a line drive or a fly ball. Table 1 (data taken from a discussion at Fangraphs) shows that ground balls result in a much lower slugging average than either line drives or fly balls.


Table 1: Slugging Percentage By Batted Ball Type


Event

SLG

Line drive

.978

Ground ball

.220

Fly ball

.494


Futhermore, Table 2 (taken from The Hardball Times Baseball Annual 2007) indicates that a ground ball is less likely to contribute to runs than either a fly ball or a line drive. The exception would be an infield fly but those are much less common than outfield flies. Whatever way you look at it, it is clear that the ability to get batters to hit the ball on the ground is a good thing.


Table 2: Run Impact of Batted Ball Type


Event

Run Impact

Line Drive

.391

HBP

.355

Walk

.355

Outfield Fly

.192

Intentional Walk

.075

Ground ball

.045

Bunt

.021

Infield fly

-.088

Strike out

-.113


Table 3 shows how Tigers pitchers ranked in 2006 in ground ball percentage. Table 4 lists the top 20 ground ball pitchers in the American League in 2006. Kenny Rogers (50%), Jeremy Bonderman (48%) and Nate Robertson (47%) were in the top 12 among 56 American League pitchers with 17 or more starts in 2006. Justin Verlander (42%), on the other hand had a below average ground ball rate. Zach Miner (47%), the next most frequent Tigers starter, had the same ground ball rate as Robertson.


Table 3: Ground ball Rates for Tigers Starters in 2006


GB% Rank

Name

IP

GB%

ERA

FIP ERA

7

Rogers

204.0

0.50

3.84

4.81

10

Bonderman

214.0

0.48

4.08

3.41

12

Robertson

208.7

0.47

3.84

4.84

36

Verlander

186.0

0.42

3.63

4.47

---

Ledezma

60.3

0.34

3.58

4.30

---

Maroth

53.7

0.42

4.19

5.99

---

Miner

93.0

0.47

4.84

4.57


*Note: These FIP ERAs include HBP and are slightly different from those posted earlier.


Table 4: Top 20 Ground ball Rates in American League in 2006


GB% Rank

Name

Team

IP

GB%

ERA

FIP ERA

1

Wang

NYA

218.0

0.63

3.63

4.03

2

Westbrook

CLE

211.3

0.61

4.17

4.00

3

Hernandez

SEA

191.0

0.58

4.52

4.03

4

Halladay

TOR

220.0

0.57

3.19

3.72

5

Janssen

TOR

94.0

0.53

5.07

4.89

6

Burnett

TOR

135.7

0.51

3.98

3.91

7

Rogers

DET

204.0

0.50

3.84

4.81

8

Bedard

BAL

196.3

0.49

3.76

3.72

9

Loewen

BAL

112.3

0.49

5.37

4.32

10

Bonderman

DET

214.0

0.48

4.08

3.41

11

Pineiro

SEA

165.7

0.48

6.36

5.36

12

Robertson

DET

208.7

0.47

3.84

4.84

13

Fossum

TB

130.0

0.46

5.33

5.45

14

Haren

OAK

223.0

0.45

4.12

4.24

15

Beckett

BOS

204.7

0.45

5.01

5.24

16

Sabathia

CLE

192.7

0.45

3.22

3.43

17

Escobar

LAA

189.3

0.45

3.61

3.74

18

Contreras

CHA

196.0

0.45

4.27

4.22

19

Millwood

TEX

215.0

0.45

4.52

4.00

20

Redman

KC

167.0

0.44

5.71

5.11

*Note: These FIP ERAs include HBP and might be slightly different from those posted earlier.

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