Herb G - OPS+ + Adjusted Park Factor

Upon reading the article on Sabermetrics in yesterday's Mack's Mets, I became intrigued with the concept of OPS+. For years now, it has been acknowledged that OPS (the sum of On Base Percentage + Slugging Average) is an important measure of a players offensive performance. I've been troubled, however, by the fact that OPS is distorted by where the player plays the majority of his games. Those who play in hitter's parks have a distinct advantage over those who play in pitcher's parks.

The idea of OPS+ is to level the playing field, so to speak. OPS+ takes into account the performance of each player relative to the average of the league he plays in, (AL or NL) and then modifies that number by the ease or difficulty of hitting in a particular home park. Using OPS+ is an important step forward in assessing the relative offensive worth of one player against another because it removes the bias of the players home park. You would expect a player who plays all his home games in Chase Field to have a higher OPS than a player with the same abilities who plays his home games in Citifield, but how much better? By actually determining how much better or worse each ball park is than the others, OPS+ lets you look at various players more objectively.

Upon research, it turns out that, although yesterday's piece mentions Ball Park Factor, the formula quoted is slightly off, because it omitted the "Ball Park Factor" in the equation. The actual formula is as follows:

• OPS+ = 100 * ( OBP/lgOBP + SLG/lgSLG - 1)/BPF (Source: http://d1prospect.com/sabermetrics.php )

where lgOBP and lgSLG are the averages of On Base Percentage and Slugging Average for the entire league, and BPF (Ball Park Factor) is a measure of the offensive production of the players home ball park.

The following table shows the various Ball Park Factors for each major league ball park for 2012.

RK	PARK NAME	RUNS	HR	H	2B	3B	BB
1	Coors Field (Denver, Colorado)	1.579	1.493	1.276	1.223	1.971	1.082
2	U.S. Cellular Field (Chicago, Illinois)	1.268	1.349	1.081	1.133	0.885	1.206
3	Fenway Park (Boston, Massachusetts)	1.206	1.088	1.173	1.494	1.389	1.036
4	Rangers Ballpark in Arlington (Arlington, Texas)	1.183	1.168	1.117	1.157	1.029	1.000
5	Oriole Park at Camden Yards (Baltimore, Maryland)	1.173	1.314	1.099	1.065	0.684	1.027
6	Chase Field (Phoenix, Arizona)	1.171	1.192	1.064	1.176	1.176	0.955
7	Miller Park (Milwaukee, Wisconsin)	1.168	1.631	0.997	0.987	1.333	1.060
8	Great American Ball Park (Cincinnati, Ohio)	1.113	1.592	1.017	0.848	1.167	1.036
9	Comerica Park (Detroit, Michigan)	1.071	1.026	1.031	0.923	1.533	0.977
10	Target Field (Minneapolis, Minnesota)	1.044	1.031	1.030	0.989	1.905	1.082
11	Turner Field (Atlanta, Georgia)	1.038	0.873	1.011	1.197	1.040	1.038
12	Kauffman Stadium (Kansas City, Missouri)	1.034	1.028	1.037	1.025	1.257	0.987
13	Wrigley Field (Chicago, Illinois)	1.024	0.962	0.981	1.077	0.930	1.134
14	Nationals Park (Washington, D.C.)	1.023	1.044	1.063	1.000	1.192	0.944
15	Rogers Centre (Toronto, Ontario)	1.008	1.030	0.992	1.075	1.312	0.914
16	Marlins Park (Miami, FL)	1.005	0.720	0.995	0.938	1.257	1.027
17	Yankee Stadium (New York, NY)	0.992	1.143	0.927	0.881	0.423	1.008
18	Busch Stadium (St. Louis, Missouri)	0.985	0.915	1.047	1.137	0.816	0.950
19	Citizens Bank Park (Philadelphia, Pennsylvania)	0.968	1.087	0.952	0.989	0.710	1.080
20	Minute Maid Park (Houston, Texas)	0.937	0.981	0.968	0.954	0.914	1.047
21	Progressive Field (Cleveland, Ohio)	0.899	0.925	0.985	1.038	0.326	0.957
22	O.co Coliseum (Oakland, California)	0.888	0.859	0.927	0.837	1.091	1.016
23	Citi Field (New York, NY)	0.874	1.069	0.918	0.785	0.629	0.966
	Tropicana Field (St. Petersburg, Florida)	0.874	0.774	0.914	0.922	1.286	1.004
25	Dodger Stadium (Los Angeles, California)	0.867	1.125	0.972	0.824	0.486	0.973
26	Petco Park (San Diego, California)	0.854	0.626	0.960	0.967	1.257	1.004
27	Angel Stadium of Anaheim (Anaheim, California)	0.812	0.759	0.906	0.909	1.095	0.769
28	PNC Park (Pittsburgh, Pennsylvania)	0.764	0.631	0.871	1.037	0.718	0.796
29	AT&T Park (San Francisco, California)	0.737	0.522	0.901	0.893	0.808	1.021
30	Safeco Field (Seattle, Washington)	0.687	0.583	0.831	0.709	0.528	1.002

Glossary

Park Factor compares the rate of stats at home vs. the rate of stats on the road. A rate higher than 1.000 favors the hitter. Below 1.000 favors the pitcher. Teams with home games in multiple stadiums list aggregate Park Factors.

Source of table: http://espn.go.com/mlb/stats/parkfactor