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