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.

4 Responses to “Some More on O-Swing%”

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  1. Colin Wyers says:

    Is that controlling for park?

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    • The O-Swing% is not controlled for park. I’m not really aware at how BIS deals with camera angles and what not when it comes to pitch plotting, so there may very well be some park bias.

      There are some very obvious park biases in the 2007 data for the Angels and Dodgers, but I’m not seeing anything that obvious in the other seasons, but I haven’t taken any in depth looks.

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      • Colin Wyers says:

        Thinking about this some more – in the aggregate a park-to-park bias is going to increase, not decrease, year to year correlation. That is to say, most players don’t switch parks year over year, and so anything that repeats itself from season to season (and the most obvious source of park to park bias in these numbers is camera position, which is consistent) should increase the correlation, not decrease it.

        In other words I don’t know that these year-to-year numbers are telling us anything about the reliability of the data on a per-player basis.

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  2. Newcomer says:

    I appreciate the OSAA, but I’m also curious how much of the other correlations would be strengthened by comparisons to league average. In other words, does pitches per plate appearance above average correlate stronger year-to-year than pitches per plate appearance itself? This might have to do with changes in the pool of pitchers in the league or, perhaps, changes in the called strike zone. These changes would be expected to affect O-Swing% similarly, while OSAA and pAA/PA would correlate similarly. This could help isolate whether we’re seeing the effects of PitchF/X system bias or the effects of pitching and umpiring changes.

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