Saturday, December 30, 2023
Introducing Predictive wOBA
Hitting a 450 foot HR is very indicative of a batter's talent. It shows that he has raw power and it shows that he can really put the barrel on the ball.
Hitting a 110 mph high popup to an outfielder for an easy out is also a good indication of a batter's talent. It shows that he has raw power and that a small mistiming is what kept him from hitting a 450 foot HR. This is what is called a Major League Out. For that particular PLAY, an out is an out, and is always bad. For that particular PLAYER, a Major League Out is almost a HR.
Similarly, a Texas Leaguer that clears the infield and lands in front of the outfield is always good for that PLAY, it is not a good indication of talent for that PLAYER.
So, let's talk about Expected wOBA and Predictive wOBA. Expected wOBA is the expected value of that PLAY. A Texas Leaguer is going to have a near 100% hit probability, and so if you have a low launch speed and a high launch angle, you are going to get an Expected Hit Probability that will approach 100%. It will explain that PLAY in RETROSPECT. It is much (much much) better to think of Expected Value in Retrospect, as in: The Expected Hit Probability WAS... Obviously, the word Expected can be used in both backward (expected was) and forward (expected will be), and so is confusing and ambiguous. The x-stats that you see, whether at Savant or anywhere else on the Web, are almost always meant to be retrospective, and simply measuring the PLAY.
Predictive wOBA is different. Predictive wOBA is tied to the PLAYER and not the PLAY. This is a critical distinction to make. A Major League Out is a much better outcome in describing the talent of a player than a Texas Leaguer Hit. When you see a Major League Out, as a fan, you should be disappointed, but as a scout, you should be elated. And the reverse when you see a Texas Leaguer Hit. The Expected wOBA (describing the PLAY) and Predictive wOBA (describing the PLAYER) is what you need to constantly remind you.
Let me show you three charts (click to embiggen). The first maps the current season's actual wOBA (actually it's wOBAcon, since we are only looking at batted balls, or Contacted plate appearances) to next season's wOBA (wOBAcon actually), for the 2020-2023 seasons, minimum 100 batted balls. Why do we compare to next season's wOBA? Because that is a (mostly) unbiased estimate of a batter's talent. That is a correlation of almost r=.5, which is pretty good. You can see that the slope of the estimate is close to .5, which means that next season's wOBA can be estimated as half-way between the current season's wOBA and the league average. So, a HR (wOBA of 2.000) in 2022 will indicate a wOBA of 1.180 of talent in 2023 (half-way between 2.000 and league average of around .360).
As we know, Actual Outcomes are filled with vagaries of the fielders and the park and the ball and on and on. This is why we prefer Expected wOBA over Actual wOBA. Expected wOBA is focused on those launch characteristics most in control of the batter (launch angle and speed), without worrying about whether the ball carries for 300 feet or 320 feet, or pulled at 20 degrees or 30 degrees, or how good the fielding alignment is positioned or how well the fielder reads the ball. All of those variables is what turns an Expected wOBA into an Actual wOBA. And Expected wOBA describes a batter's talent better than Actual wOBA, which you can see by the correlation having an r above .55. The slope of the line also suggests that you would weight the Expected value at 58%, and the league average at 42%. Every combination of Launch Angle (from minus 90 to plus 90) and Speed (from 0 to 125) has a distinct Expected wOBA value. The chart is obviously massive at 181 x 126 entries.
Now we come to the star of the show, Predictive wOBA. How does it work? We break up a batter's launch characteristics of each play, first along Launch Angle, into three categories. We have the Ideal Launch Angle, the Sweetspot range, of 8 to 32 degrees. We have launch angles above 32 and launch angles below 8 degrees.
Then we break up a batter's Launch Speed into four categories: 105+, 100 to 104.999, 95 to 99.999, and under 95 mph.
This gives us 12 combinations of speed and angle. For each combination, we get a Predictive Value (analogous to Expected Value, but in terms of the PLAYER, and not the PLAY). Here are those values.
So, when a batter gets his Major League Out, or any batted ball at over 32 degrees of launch at 100+ mph, that has a Predictive wOBA value of .838. This is one of the BEST things a batter can do, as it indicates TALENT. That's what Predictions are: an estimate of the TRUE TALENT of a PLAYER. That perfect hit, a ball hit at the Ideal Launch Angle of 8 to 32 degrees, at 105+ MPH: that is only SLIGHTLY more indicative of talent than a Major League Out: that has a Predictive wOBA value of .867.
The worst thing a batter can do is a high launch angle and low-speed, as that has a true talent value, a Predictive wOBA value of .206.
Once we apply the Predictive wOBA on each batted ball for every player and aggregate it at the season level, we can then compare to the next-season's Actual wOBA. And this is what we get: a correlation of r=0.61. Indeed, if you include all three measures, the Actual wOBA, the Expected wOBA and the Predictive wOBA, we STILL get an r=0.61 to next season's wOBA. This suggests that creating these 12 bins (it's actually 8, as there are some bins that stretch beyond one bin) is sufficient to describe a batter's profile. And we can completely ignore a batter's Actual wOBA as well as their Expected wOBA.
Can you improve this? The next step really to make it truly predictive is to also incorporate the amount of batted balls in the sample. The higher the number, the more indicative the outcomes are. But, we'll save that for a future thread.
So, we no longer need to compare Expected wOBA to Actual wOBA and talk about luck or Random Variation as being the distinguishing feature as to why the Expected wOBA diverged from Actual wOBA. No. What we actually care about is Predictive wOBA, and we don't even care about Actual wOBA any more, not if we care about the True Talent of our batter.
Next time, I'll repeat this process for Pitchers. I haven't done it yet, so I'm just as curious as you are.
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