Are there issues with the 2007 data corrections?
Recently at the PITCHf/x summit Ike Hall presented a talk on data corrections where he noted that it appears that my corrections are over correcting the data on off speed pitches. You can find the talk on the summit's website it is labeled Data_Improvement.pdf. On page 11 Ike shows a plot of the differences between the drag coefficient at Comerica and PETCO parks. While the data seems to great in the fastball region it appears to differ from 0.3 to 0.5 for the drag coefficient for pitches thrown near 60 MPH.
Now this seems like a huge issue. Obviously there a very large difference between 0.3 and 0.5. The problem is difference in drag actually results in a very small effect. If you assume the air density to be 1.2 pascals, the balls initial velocity to be 60 MPH (26.8 m/s), the circumference of a baseball to be 9 inches (area 0.004 m^2), then you can calculate the drag force. If you do that you get the drag force between and 0.52 and 0.86 N. If you then want to say want to find the differences in final velocity you can use the equations of motion and if you assume the ball takes 0.5 second (which is actually quite large) you get a difference in final velocities of less than a third of a meter per second.
So while it appears that my corrections are indeed over correcting the data the results of these over corrections are small. That said, I will be looking in to how to adjust for this to fix my corrections but don't expect a huge change to the data. Once I get that worked out I should be ready to run the corrections on the 2008 data as inter league play is nearly upon us and that is what my code really needs.

1 Comments:
Josh, you are right that it doesn't make a huge difference in velocities for those slower pitches, but it does give a systematic difference between those parks in terms of break. If the drag coefficient is off it means that we are most likely mis-measuring at least two accelerations by about the same fraction because at least one camera was mis-calibrated in some way...most likely with it's spherical distortion. In the case of petco, it was the y and z directions that were primarily affected, and by a large amount. This interplay of more than one direction can affect the break number we measure systematically by 2-4 inches for slower pitches like curveballs. If you were to look at a pitcher and determine that his curveball was breaking 2-4 inches more or less than it normally does, many people would point to that as evidence that he was doing something different during the game.
So being off with drag means that we are also off with the other forces on the ball as well...
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