Tuesday, June 03, 2014
Situational Wins, one more time
Studes does a great job of laying out various scenarios for how Situational Wins (WPA/LI) works.
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The key to understanding the metric is that every PA is equally weighted. So, whether it's a blowout scenario, or bottom of 9th tied game scenario, THE MEAN AND THE SPREAD are similar. This is why, as studes astutely notes that the value of the out is the same in basically all scenarios. (Not entirely true with runner on third and less than 2 outs, but let's leave that aside for us, and for studes to contemplate.)
In essence, the average win value of a negative event is around -.027 wins, and the average win value of a positive event is around +.054 wins. If you have a league average OBP of .333, you can see how this balances perfectly. (For the rest of this blog post, I'll use -.025 and +.050, so it makes it easier to do the calculations.)
In a very high leverage situation (say an LI of 5.0), the average win value of a negative event is -.025 x 5 = -.125 and the average win value of a positive event is +.050 x 5 = +.250. The RELATIONSHIP between the negative and positive win values remains constant. It has to, because the average value has to be zero (i.e., .667 x whateverNegativeValue + .333 x whateverPositiveValue always equals zero). The .333 is the OBP and the .667 is 1-OBP.
So, we divide those win values (-.125, +.250) by the LI to get it back onto the normal scale, so that each PA is equally weighted, regardless of high and low leverage.
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With me so far? Ok, now that we know how to keep those things in balance, the next step is to try to keep the positive events in balance. It would help to think in terms of wOBA, but let's not for now.
The average positive event is +.050 wins. The walk is +.03, the single is +.045, the HR is +.13 wins, etc. On average. But when 1-run is needed, the win value of the single goes up, while the win value of the HR goes down. It also depends on bases and out situations, along with inning and score. The key though is that they all are CENTRED around +.050 wins.
In the most extreme scenario, where a walk or HR leads to the same result (walkoff win), ALL positive events will equal +.050 wins. Exactly the same for every positive event. And it's going to be double (and reverse the sign) of the win value of the out. (As long as the OBP is .333.)
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So, that's what WPA/LI does:
(a) it forces the "range" of the positive and negative events to be the same, regardless of inning, score, base, out, meaning negatives average -.025 and ?positives average +.050.
(b) it forces each positive event to reflect their impact for the particular inning, score, base, out
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WPA/LI is a metric that really rises above all other metrics in terms of understanding exactly what is happening. Unfortunately, it's not easy to explain. Hopefully, studes' terrific piece adds a few more people onto the Situational Wins train.
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