Is the BIS data right?

The Fan Graphs website displays a full plate of baseball statistics ranging from traditional counting numbers to sabermetrics to scouting data. Their scouting data comes from the Baseball Info Solutions (BIS) company which serves Major League Baseball teams and publishers of baseball statistics like Fan Graphs. In an article on Fan Graphs by author Joe Pawlikowski, Joe finds something interesting in the very popular plate discipline statistics section:

While overall swing percentage isn’t too far off from previous years, both O-Swing% and Z-Swing% have moved moved a bit. This year hitters are swinging at 28 percent of pitches outside the zone, a nearly three point jump from 2009. The number does move around a bit, dipping as low as 16.6 percent in 2004.

What Joe is talking about is the average of the metric O-Swing% which is basically the percentage of swings on pitches outside the strike zone. Joe finds that the average O-Swing% for a season has steadily gone up from when it was first tracked in 2002 to the current 2010 season. He also presents six ideas in the article on why this may be happening. They are all very possible answers either as a sole reason or a combination of the six. However the number five idea takes a stab at the data itself.

5. The criteria for pitches inside and outside the zone has changed.

With the pitch f/x data, we now have something to compare the data to. Unfortunately, pitch f/x only started capturing a full season’s worth of data in 2008 so we can’t compared anything prior to 2008. But we can see whether or not that 3% percent increase in O-Swing% this season is real or a change in the strike zone definition. So here is the O-Swing% from Fan Graphs along with the O-Swing% from pitch f/x for a 20 inch wide plate (approximate length of home plate plus the radius of the baseball) and the called strike zone as defined here.

According to the pitch f/x data, it seems like O-Swing% is actually down from 2008 just slightly. So it is very likely that either BIS changed the criteria for defining the strike zone or it is a unwanted change caused by the susceptibility of producing their data from video by the television broadcasts.

Hat tip to Nick Steiner for the idea for this post.