For those of you who have been reading baseball content at Check Down Sports semi-regularly, you’ve probably seen one of us talking about players and teams we think are performing at a level far from expected. A lot of times when attempting to explain the reasoning behind abnormal pitching performance, we cite a few reasons, and then attribute the rest to good or bad luck. Luck we usually associate with a batter’s batting average on balls in play (BABIP), which is agreed upon by most as beyond the control of the pitcher.
The influx of ball tracking systems in MLB has allowed for a boatload of new measurements that, until a few years ago, were only dreams in the minds of analysts and evaluators. One of those– the velocity of ball exiting the bat (exit velocity)– is a popular, yet informative piece of data.
Intuitively, it makes sense that the softer the ball leaves the bat, the less likely the ball should result in a hit. A pitcher who suppresses exit velocity should allow fewer batted balls to become base hits than a pitcher who gives up a high exit velocity. Yes, bloops and seeing-eye ground balls will find open space, but on average, I think this assumption makes sense.
But thanks to Statcast and baseballsavant.com, this assumption doesn’t have to be an assumption at all. We can test it out.
Baseball Savant has exit velocity data since the beginning of 2015, so that’s where I started. I gathered average exit velocity against for pitchers with at least 190 batted-ball events in 2015 and 2016 (298 total). I then got the BABIP for those pitchers in those seasons from FanGraphs. Next, using STATA, I ran a simple linear regression with the two variables. Results are shown below.
The scary math-stuff explained:
- A pitcher’s BABIP isn’t entirely caused by luck
- Exit velocity has a minor, yet significant, effect on BABIP
- 6% of a pitcher’s BABIP can be explained by exit velocity
- If a pitcher decreases his average exit velocity by 1 mph his BABIP will decrease by 0.005 points, on average (i.e. a pitcher decreases his average exit velocity from 90 to 89 mph– his .300 BABIP would fall to .295. In turn, this would lower his ERA).
- The bottom-left quadrant is ideal. Though, because of exit velocities’ small effect on BABIP, probably not sustainable. We’ve seen Arrieta and and Chris Young come back to earth a bit in 2016.
- The top-left quadrant includes candidates for improvement in the second-half of 2016 or 2017. Pitchers here have been unlucky in terms of BABIP. Their exit velocities suggest they should have a lower BABIP, and therefore, ERA.