Fans and students of baseball are continually devising new ways to analyze the game. There's a name for this science: sabermetrics, derived from SABR, the Society for American Baseball Research. Among the many statistical and analytical tools dreamed up by sabermetricians is "PFX value," which tracks how much a pitched ball deviates from a straight path between the time the pitcher releases it and the time it reaches home plate.
Gameday
MLB.com, the website of Major League Baseball, includes an application called "Gameday," which allows users to follow the progress of a game with a wealth of data presented in near-real time. Gameday's features include a strike-zone diagram that shows the location of each pitch when it reaches the plate. The diagram also identifies whether the pitch was a called ball, a called strike, a swinging strike, a foul ball or a ball hit into play.
PITCHf/x
At one time, Gameday pitch locations were entered manually. Observers working for MLB.com watched each pitch and estimated its position relative to the strike zone. No matter how diligent their efforts, their work remained subjective and prone to observation error. Further, this system produced only one data point -- the position of the ball when it reached home plate, with no information on what happened before it got there. Starting in 2007, Major League Baseball installed a system called "PITCHf/x" in all 30 big-league ballparks. The system, devised by Sportvision Inc., uses cameras to track the flight of the ball from the moment it leaves the pitcher's hand until it reaches the plate. A computer program uses the information from the cameras to determine the point at which the ball reaches the plate, accurate to within a half-inch, and it feeds that information to Gameday, which uses it to plot pitches on the strike-zone diagram.
PFX Value
PITCHf/x produces a lot more data about a pitch than just whether it is in or out of the strike zone. The cameras also identify the point at which the pitcher releases the ball, the trajectory of the pitch and the speed of the pitch -- both when it leaves the pitcher's hand and when it reaches the plate. MLB.com makes all this data available to sabermetricians, who have used it to calculate PFX value. Simply put, PFX tells you how much movement the pitcher puts on the ball. Based on release point, speed, trajectory, and gravity, a ball that a pitcher has thrown "straight" should cross the plate at a certain point. But if the pitcher has put an unusual spin on the ball, as with a sinker, or no spin, as with a knuckle ball, air resistance will cause the pitch to deviate from that straight path. PFX measures the distance between the point at which the ball "should" have crossed the plate, if thrown straight, and the point at which actually crossed the plate.
BRK Value
A related concept to PFX is BRK, or "break value" of a pitch, which can also be derived from PITCHf/x data. A perfectly thrown curve ball actually might end up crossing the plate at the same position as a straight ball, because such a pitch swoops outward, making the hitter think it's going to be out of the strike zone, and then curves back into the zone. As a result, it would have a PFX value close to that of a ball that didn't curve at all. BRK, on the other hand, measures the pitch's maximum deviation from the straight path, regardless of where it ended. The higher the BRK, the farther the pitch strayed from "straight" at some point in its flight.
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