October 23, 2010

BP Unfiltered

PitchTrax Wrong, Not Ump

by Mike Fast

Printer-friendly

Contact Author

Normally the PitchTrax data shown on television is pretty accurate, within an inch or so of reality.  It's based on Sportvision's PITCHf/x pitch-tracking camera system, which usually does a great job.  Not so in Friday's game.  You might have thought home plate umpire Brian Gorman had lost his bearings around home plate.  However, in this case it was the PITCHf/x camera that was lost.  According to my calculations, the horizontal plate locations reported by PITCHf/x in Game 6 of the American League Championship Series were off by about 5-6 inches toward the third base side of the plate.  

I calculate this offset by comparing where pitchers have located their pitches to left-handed and right-handed batters throughout the past few seasons to where the pitchers located their pitches in the game under consideration.  For one pitcher, the comparison may not be very helpful, but when aggregated across all the pitchers who appeared in a game, this method tends to be fairly accurate, within an inch or two at the game level, and better than that if you average a few games together.

Here are all the horizontal offsets calculated by this method for each League Championship Series.

The PITCHf/x system in Philadelphia seems to be the only one that's really well calibrated with regard to plate location.  The system in San Francisco was off about two inches, which is quite a lot.  The system in Yankee Stadium was off about an inch, which isn't great but is more within the realm of normal operation.  But the system in Texas, which wasn't calibrated particularly well in the first two games of the series, went positively haywire in Game 6.  

This is actually the fourth-worst excursion observed at a single-game level in the 2008-2010 time frame.  Some of those were probably statistical flukes from my measurement method, but the strike zone called by the umpire tonight suggests that this was not a statistical fluke.

PITCHf/x plate locations, and thus the television PitchTrax locations, really were off by about 5-6 inches in Friday's Game 6.  Please retract all tomatoes thrown at Brian Gorman on the basis of the PitchTrax data.

EDIT: Corrected the name of the home plate umpire.

Mike Fast is an author of Baseball Prospectus. 
Click here to see Mike's other articles. You can contact Mike by clicking here