1/8/13

Herb G - OPS+ + Adjusted Park Factor

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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:
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.
RKPARK NAMERUNSHRH2B3BBB
1Coors Field (Denver, Colorado)1.5791.4931.2761.2231.9711.082
2U.S. Cellular Field (Chicago, Illinois)1.2681.3491.0811.1330.8851.206
3Fenway Park (Boston, Massachusetts)1.2061.0881.1731.4941.3891.036
4Rangers Ballpark in Arlington (Arlington, Texas)1.1831.1681.1171.1571.0291.000
5Oriole Park at Camden Yards (Baltimore, Maryland)1.1731.3141.0991.0650.6841.027
6Chase Field (Phoenix, Arizona)1.1711.1921.0641.1761.1760.955
7Miller Park (Milwaukee, Wisconsin)1.1681.6310.9970.9871.3331.060
8Great American Ball Park (Cincinnati, Ohio)1.1131.5921.0170.8481.1671.036
9Comerica Park (Detroit, Michigan)1.0711.0261.0310.9231.5330.977
10Target Field (Minneapolis, Minnesota)1.0441.0311.0300.9891.9051.082
11Turner Field (Atlanta, Georgia)1.0380.8731.0111.1971.0401.038
12Kauffman Stadium (Kansas City, Missouri)1.0341.0281.0371.0251.2570.987
13Wrigley Field (Chicago, Illinois)1.0240.9620.9811.0770.9301.134
14Nationals Park (Washington, D.C.)1.0231.0441.0631.0001.1920.944
15Rogers Centre (Toronto, Ontario)1.0081.0300.9921.0751.3120.914
16Marlins Park (Miami, FL)1.0050.7200.9950.9381.2571.027
17Yankee Stadium (New York, NY)0.9921.1430.9270.8810.4231.008
18Busch Stadium (St. Louis, Missouri)0.9850.9151.0471.1370.8160.950
19Citizens Bank Park (Philadelphia, Pennsylvania)0.9681.0870.9520.9890.7101.080
20Minute Maid Park (Houston, Texas)0.9370.9810.9680.9540.9141.047
21Progressive Field (Cleveland, Ohio)0.8990.9250.9851.0380.3260.957
22O.co Coliseum (Oakland, California)0.8880.8590.9270.8371.0911.016
23Citi Field (New York, NY)0.8741.0690.9180.7850.6290.966
Tropicana Field (St. Petersburg, Florida)0.8740.7740.9140.9221.2861.004
25Dodger Stadium (Los Angeles, California)0.8671.1250.9720.8240.4860.973
26Petco Park (San Diego, California)0.8540.6260.9600.9671.2571.004
27Angel Stadium of Anaheim (Anaheim, California)0.8120.7590.9060.9091.0950.769
28PNC Park (Pittsburgh, Pennsylvania)0.7640.6310.8711.0370.7180.796
29AT&T Park (San Francisco, California)0.7370.5220.9010.8930.8081.021
30Safeco Field (Seattle, Washington)0.6870.5830.8310.7090.5281.002
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

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