Wednesday, August 23, 2023
WPA by Event and wOBA
PROLOGUE / PRELUDE
As readers of The Book know, I created wOBA as the central metric for data analysis almost twenty years ago. Its basis is the run values introduced by Pete Palmer in The Hidden Game of Baseball back in the early 1980s.
The scale of wOBA is that of OBP, where naturally the floor is .000 and they share similar averages (say around .320 or .330). The run values from Pete were centered so that the average was 0, and the run value of the out was around -.25 or -.30 runs. Having average = 0 was offputting for some folks, because zero is associated to nothing, or null.
Think for example of two pitchers with a league average ERA, one with 180 IP and another with 18 IP. The 180 IP pitcher will get paid more than ten million a year, while the 18 IP pitcher might not even make the roster the next season. Both would be shown with a 0-runs-above-average, which while technically accurate is not reflective of their value if you rely one ONE number. Showing them each with a .325 wOBA, with one facing 750 batters and the other facing 75 batters is much better: it shows their value along two dimensions, and so allows for different conclusions, all depending on how you merge the two values into one.
When you are already given just the one value, be it wins above average (WAA), or wins above readily available talent (WAR), or even a player's salary, the conclusion is now forced on you. You have no way to back out of it, since the one value encapsulates everything that went into it. There's nothing left. This is why for example I would much more prefer to show the above pitchers with an Individualized Won-Loss Record, aka The Indis. If one has a 5-5 record, and the other has a 0.5-0.5 record, well, now each of us can now do something unique with that. I can compare each of those to a .300 win%. You can compare each to a .400 win%. Someone else can compare each to a .500 win%. No one is beholden to anyone. We all agree on their accomplishments, the 5-5 and the 0.5-0.5. We don't need to agree on the value of those accomplishments. That's for you to decide on your own.
WPA
I actually didn't want to talk about any of that. I wanted to talk about win probability added (WPA), or win values. Since 2010, we've had over 70,000 home runs. The win expectancy when the batter stepped up to the plate was .520. After hitting the HR, his team had a win expectancy of .653. That difference, +.133 wins, is what is called WPA. That's the win value of the HR, relative to the average (which naturally is .000). So, you can see why I had to talk about average = 0, even though that was a long-winded way to get here.
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