Get to Know: O-Swing%

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.

Links and Resources:
Dissecting Plate Discipline: Part 1
Dissecting Plate Discipline: Part 2
More on Plate Discipline
Pitchers, Pitch by Pitch
Expanding the Strike Zone