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.