A Note On O-Swing%

Just in case you find yourself playing with the Fangraphs plate discipline numbers, be sure to keep an eye on the league averages.

It's not that big of a deal for most of the statistics, since by and large they stay pretty stable, but the league average O-Swing% in particular has changed an awful lot over the years as BIS (the data source) has changed its definition of what pitches are and aren't in the strike zone.

2002: 18.1% average
2003: 22.2%
2004: 16.6%
2005: 20.3%
2006: 23.5%
2007: 25.0%
2008: 25.4%
2009: 24.4%

This is important to keep in mind if you're looking at, say, Adrian Beltre or Ichiro. Straight O-Swing% would tell you that they're swinging at twice as many balls in 2009 as they did in 2004, but that isn't true. Though they're hacking more, they're not doing so to such an extreme degree.

The best approach to dealing with this, I think, is to simply calculate the ratio of O-Swing/lgO-Swing to end up with a sort of O-Swing+. Here's a sample table for Beltre:

In 2009, Beltre has swung at 66% more balls than the league average. In 2004, he was down at 26% more.

That's all. Just something to keep in mind. It sounds pretty basic, but the Fangraphs player pages are formatted in such a way that the averages are easy to miss.