Posted by John Dewan on March 28, 2017
Earlier this month at both the Sloan Analytics Conference in Boston and the SABR Analytics Conference in Phoenix, MLBAM’s StatCast team released details on a new statistic that they have started to report on flyballs to the outfield: Catch Probability. Catch Probability is a measure of how hard a catch is to make based on how far the batted ball is from the nearest outfielder and how long it has been in the air. Here is a full explanation of how it works from MLB.com. Because of our long-time passion for defensive metrics, we at Baseball Info Solutions (BIS) are particularly interested in their new statistic, especially in how it compares to Out Ratio, which is the basis of the Range and Positioning metric that serves as the foundational component of Defensive Runs Saved.
Catch Probability and Out Ratio are similar. Both measure how hard it is to catch a specific type of ball. Both rely on hang time to factor in difficulty. However, by measuring how far the fielder has to run, Catch Probability is measuring range by itself. Out Ratio measures range and positioning taken together in that it does not factor in the starting position of the outfielder.
The two metrics have differences, but they both get at the same question of how difficult a flyball was to catch. And both methods group plays with other plays having similar measurements from recent seasons to answer that question.
The MLB.com article shows a couple of examples that we can use to compare the two systems. The first is an awesome diving catch from Billy Hamilton from the Reds’ April 26 game against the Mets last season in the bottom of the fifth inning:
Catch Probability gives that flyball only a 7 percent chance of being caught. Hamilton had a long way to run on this play. Out Ratio gives center fielders a 35 percent chance of making the play. Catch Probability is just trying to measure range and ignores positioning.
Next up is Matt Kemp, who made his own diving catch last year on April 5 versus the Dodgers in the top of the fourth inning.
Similarly, Out Ratio is higher than Catch Probability. StatCast gives the flyball a 75 percent chance of being caught. Meanwhile, Out Ratio pegs the flyball at 100 percent; all 33 flyballs hit to this location with the same hang time from the last two years have been caught. Kemp’s positioning made it a tougher play.
As you can see from these two examples, bad positioning makes plays more difficult than they would be for outfielders if they simply had to rely on their range. This is as we would expect. Positioning has always been important in baseball, even more so in recent years. But while these two examples show instances of outfielders being positioned a bit worse than the average Out Ratio suggests, we would actually expect fielders to be positioned better than Out Ratio’s average more often than not. And so Catch Probability from StatCast is generally going to be higher than Out Ratio from BIS because of that advantage of positioning.
We look forward to more excellent work like this from the StatCast group. The kind of in-depth data the StatCast system collects will allow for measurement of defensive skills in finer and finer detail. Effectively splitting range and positioning into separate metrics is just the start. We are getting close to a day when we can tell you not only how much range a fielder has, but how quickly he reads the ball off the bat, how quick his first step is, how fast he is on approach, how well he takes routes to balls, and how sure-handed he is; and we will be able to hang a Runs Saved number on each part. The summation of all of those individual parts may not be too different from the numbers you’ve seen for years, but the level of detail will be incredible. For baseball analytics fans, the more the merrier.