Some More on O-Swing%

Yesterday, Joe Pawlikowski noted that there seems to be an increase in the overall O-Swing% so far this season, which led to some questions about whether the strike zone was being measured consistently from season to season.

Over the course of the nine years Baseball Info Solutions has plotted pitches, there have been some not so small changes in the average O-Swing%. Over the past 3 years the numbers have been stable, but this season it seems O-Swing% is up about 3%. This can sometimes make raw O-Swing% a difficult stat to match up to year to year because the baselines can be somewhat different.

However, when looking at a player’s O-Swing% above average, there is a very strong correlation from year to year and this continues to be the case for the 2010 data. In other words, a typical player’s “plate discipline” does not end up changing that much season to season. Here are the more recent correlations for players with greater than 50 plate appearances compared to BB% or Pitches/PA.

O-Swing% Above Average / BB% / Pitches per PA
2009 – 2010 – .74 / .56 / .66
2008 – 2009 – .74 / .64 / .68
2008 – 2010 – .68 / .53 / .57

As you can see, even when compared to something as seemingly stable as Pitches/PA, O-Swing% is definitely more stable from year to year once you adjust for the baseline.

So the lesson here is that average O-Swing% is important to take into consideration. We’ll be adding O-Swing% Above Average (OSAA for short) to our repertoire of stats starting tomorrow, which will make life somewhat easier when comparing a player’s O-Swing% from season to season.

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David Appelman is the creator of FanGraphs.

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