O-Swing% (outside swing percentage): The percentage of pitches a batter swings at outside the strike zone.
Why you should care: When a batter swings at a pitch thrown outside the strike zone his chances of success are severely decreased. The ability of a batter to differentiate pitches inside or outside the strike zone is often referred to as plate discipline and O-Swing% is a good measure of true plate discipline. Likewise, pitchers try to get batters to swing at pitches outside the strike zone and O-Swing% for pitchers shows how adept a pitcher is at tricking batters to swing at pitches outside the strike zone.
Current Baselines: The major league average O-Swing% changes slightly from year to year. The chart below shows the major league averages from 2005-2008 for both batters and pitchers.
2002 18.1% 2003 22.2% 2004 16.6% 2005 20.3% 2006 23.5% 2007 25.0% 2008 25.4%
Batters: A lower O-Swing% is preferable for batters.
Pitchers: A higher O-Swing% is preferable for pitchers.
David Appelman is the creator of FanGraphs.