Catch Probability represents the likelihood that a batted ball to the outfield will be caught. With Statcast tracking the exact start position on the field for each fielder and also measuring the hang time of each batted ball, the two most important pieces of data to define the difficulty of a catch opportunity are: 1. How far did the fielder have to go? 2. How much time did he have to get there?
Accordingly, each tracked batted ball to the outfield is assigned an expected Catch Probability percentage -- relative to comparable catch opportunities in the Statcast era -- based on distance needed and opportunity time. The more time a fielder has to react to a ball and the less distance needed to reach it, the higher the Catch Probability.
Distance needed is used instead of distance covered, which measures the ground a fielder covers from the point the ball is struck to the time the play is made, to prevent fielders from receiving additional credit for taking longer-than-necessary routes to the ball. Meanwhile, opportunity time is calculated from the point the ball leaves the pitcher's hand (rather than when the batter makes contact) to credit instances in which a fielder adjusts his position based on where the catcher sets up or what type of pitch is called.
As of May 2017: Catch Probability began to account for fielding direction, with plays compelling an outfielder to travel in the opposite direction of home plate (colloquially described as "going back on the ball") receiving an adjustment to account for their relative degree of difficulty.
For the purpose of determining what counts as "going back on the ball," the 360 degree range around each fielder is cut into six 60 degree sections (0 degrees is always toward home plate, while 180 degrees is always straight back). The zone defined as "going back" is 30 degrees to the left and right of the 180 degree line.
Note: Catch Probability is not available for infielders and does not account for the extra difficulty of plays made at or near the wall. However, these factors may become incorporated in future updates of the model.
Why it's useful
Catch Probability attempts to quantify outfield defense by measuring the difficulty of a particular catch. Though two unique plays may be marked as outs in the scorebook, a different amount of skill is required to catch a lazy fly ball as opposed to a sinking liner. Instead of relying on the eye test, fans can use Catch Probability to credit that difference in difficulty accordingly.
Based on the ability to quantify each play's Catch Probability, the following star ratings were created. Plays with a Catch Probability higher than 95 percent don't merit a star rating.
0 to 25 percent Catch Probability -- 5 Star play
26 to 50 percent -- 4 Star play
51 to 75 percent -- 3 Star play
76 to 90 percent -- 2 Star play
91 to 95 percent -- 1 Star play
Note: The chart above shows how Catch Probability is determined based on distance needed and opportunity time.
Watch: Statcast measures the Catch Probability on Kevin Kiermaier's terrific grab.