Monday, January 14, 2019
Of Spray Angles, FIP, and xWOBA
FIP is one of the most enduring metrics in the last 25 years. I created it in the same way that Bittersweet Symphony is the best song The Rolling Stones created in the last 25 years. FIP is really DIPS but consumable. Had someone told Voros: can you give me a quick version of DIPS, quick-DIPS, he would have created FIP. He didn't, but really, he would have, which is why I give 99% credit of FIP to Voros. I just provided the five second catchy beat that loops through Bittersweet Symphony.
Speaking of which, listen to this while you keep reading. It's going to get boring, so you'll need something to keep you awake:
FIP came to the forefront because, by itself current year FIP predicted next year's ERA (FutureERA) better than current year ERA. This was the groundbreaking part of DIPS.
Interlude
Take a step back 30 more years back: We learned that current year ERA predicted future W/L record better than current year W/L record. This is why saber-leaning folks reject a pitcher's W/L record: when the thing you are interested in is better measured by something other than itself, this is a sign that the thing you are interested in is filled with noise. And it's easily explainable: half the W/L record is actually the team's offense. Another portion is the team fielding, and the team bullpen also helps. Suddenly, the pitcher only explains a third of his own W/L record.
FIP showed similar tendencies to better predict future ERA than ERA itself. Which means we can use FIP to predict a pitcher W/L record better than either ERA or the W/L record itself.
What is FIP?
The bad part: because it did so well, people started calling FIP a metric about "what could have happened" and not "what did happen". They are wrong. All of them. FIP is 100% about WHAT DID HAPPEN, or at least a slice of what did happen, in the same way that OBP is a slice of what did happen. Neither tells the whole story of what did happen. But because FIP tells a better story of what will happen than OBP does, FIP was TREATED as "future FIP". It is not.
At its core ACTUAL FIP is:
- 13*HR + 3*BB - 2*SO
In other words, walks have a bit more run impact than strikeouts, and HR have more than 4x the impact of walks.
Future FIP
And as a DEFACTO future-FIP, it works well. But if I were to create a FUTURE FIP for the community to use, it would look more like:
- 6*HR + 2*BB - 3*SO
In other words, a strikeout tells you more than a walk about future runs, a reverse of "what did happen". HR only tell you 3x what walks tell you, and not the 4x "what did happen" would suggest. This particular FUTURE FIP is for illustration purposes only, but it would be along those lines conceptually.
Glenn DuPaul realized all this when he introduced Predictive FIP seven years ago:
FIP is a descriptive statistic that works fairly well as a predictive statistic, not the other way around.
And this is the core of Predictive FIP:
- (7*HR)+(1.6*BB)-(2*K)
If you look at his Predictive FIP, all the non-K components are chopped in half. In other words, his Predictive FIP is:
- 50% FIP plus 50% K rate
***
So, what does all this have to do with Spray Angles and xWOBA? Well, the song is half-over, so, we’ll continue this in part 2.
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