Thursday, January 02, 2020
Statcast Lab: Layered wOBA
?Several weeks back, I introduced the concept of Layered Hit Probability (and by extension Layered wOBA). The objective is straightforward: we see a hit happened, and our job is to simply answer why? and how?. How much was due to the exit velocity? How much to the launch angle? The spray direction? The fielding alignment? The park configuration? The temperature? By layering the various considerations, we can then isolate all those factors.
As a first step, since the one factor that is both the most important as well as linked to the batter himself is the exit velocity, we'll start with that. Now, you may think this is an easy step: check to see how the league does at each exit velocity, use that as the baseline, and compare the batted ball to that baseline. In theory that sounds fine. But, this is what that looks like:
So what does this say? Taken literally, it would suggest not to hit the ball much harder than 112 mph, because after that point, the wOBA flattens or goes down. But, look what happens when we compare the actual wOBA to our model that uses the speed AND angle:
This might give us more insight. The model knows that the harder you hit the ball, the better the outcome... GIVEN a launch angle. So what the above shows is that the really hard hit balls, those close to 120 mph, must have alot of poor launch angles. And this makes sense: the more flush you hit the ball, you gain speed, at the expense to loft. In other words, more line drives and ground balls and fewer fly balls. What you lose in HR, you gain in GB outs. And that's why the wOBA is so low at the very high launch speeds.
What we want to do instead is "neutralize" the launch angle. We take the league-wide spread in launch angles, and apply that to each exit velocity. This way, the 120 mph bucket is made up of EXACTLY the same launch angle distribution as you'd find in the 100 mph bucket... even though we know in practice that does not happen. When we do that, when we neutralize the launch-angle, now we see the effect, the relationship of wOBA to exit velocity. And here we see that starting at around 85mph, wOBA ALWAYS goes up as exit velocity goes up.
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And this angle-neutralized wOBA is our baseline, what we will compare our actual wOBA against. And if you find that your actual wOBA is below this baseline, which you can see happens at 120 mph, then we can easily explain it by the next layer: your launch angle wasn't good enough.
Stay tuned, as my next post will give us leaderboards for the Exit Velocity component in the Layered wOBA.
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