Statcast recently rolled out an adjustment to its ground ball xwOBA model to account for batter speed, and I set out to test how well that adjustment was doing. I used 2018 data for players with at least 100 batted balls (n=390). To get a proxy for sprint speed, I used the average difference between the speed-unadjusted xwOBA and the speed-adjusted xwOBA for ground balls. Billy Hamilton graded out fast. Welington Castillo didn’t. That’s good. Grouping the players into thirds by their speed-proxy, I got the following
|speed||Actual GB wOBA||basic xwOBA||speed-adjusted xwOBA||Actual-basic||Actual- (speed-adjusted)||n|
The slower players seem to hit the ball better on the ground according to basic xwOBA, but they still have worse actual outcomes. We can see that the fast players outperform the slow ones by 50 points in unadjusted wOBA-xwOBA and only 29 points after the speed adjustment.
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