Sunday, June 10, 2012
What a no-hitter tells us about WPA/LIPosted by Dave Studeman
I posted an idea yesterday, a made-up stat just for fun. I called it No-Hitter Added because it was a "real time" stat that apportioned credit among Mariners' pitchers for their Friday no-hitter. Tom Wilhelmsen was given a lot more credit for the no-hitter than Kevin Millwood, because Wilhelmsen pitched the critical ninth inning when the no-hitter was most in reach ... despite the fact that Millwood faced six times as many batters.
As a reminder, if you were to break out credit for the no-hitter based on batters faced (which is obviously the best way to do it), it would look like this:
K Millwood 60% T Wilhelmsen 10% S Pryor 10% C Furbush 10% B League 7% L Luetge 3%The No Hitter Added stat was just for fun, an exercise to show how WPA works, as well as its strengths and its flaws. kds posted a great comment on the article, noting that an approach that divides the No Hitter Added stats by the Leverage Index would be fairer. And that got my head spinning.
See, it's easy to calculate the Leverage Index of each batter faced in this situation, because the only outcomes we care about are a hit or not. For instance, with two out in the ninth, the Leverage Index is 1.0, because an out will result in a no-hitter (a 1) but a hit will result in no chance of a no-hitter (a 0). 1-0 equals one. In another example, the Leverage Index of the first batter is 0.0007, because a hit results in a 0, but an out leads to the next at-bat, at which point the team has a 0.07% probability of throwing a no-hitter (not giving up a hit to the next 29 batters). 0.0007 minus zero is ... well, you get the idea.
In all cases, the "No-hitter Leverage Index" of an at-bat is the probability of throwing a complete-game no-hitter starting with the next at-bat.
So now we have No Hitter Added, No-Hitter Leverage Index, and one divided by the other (NHA/LI). Let's apportion credit among Mariner pitchers for that no-hitter using NHA/LI:
K Millwood 60% T Wilhelmsen 10% S Pryor 10% C Furbush 10% B League 7% L Luetge 3%Yeah, the results are exactly the same as those using batters faced. In other words, NHA/LI works.
So what, you say? Did I just waste your time? Was that just a bag of gas for nothing? Well, yes, it mostly was except for one thing. Now you know why WPA/LI works.
Most people get WPA and LI, but WPA/LI is maddeningly difficult to explain. Tangotiger has tried several times, as have I, but it's hard to do. People have just had to take it on faith that it works.
But No Hitter Added is a much simpler concept (and the math is much simpler) than Win Probability Added. And here you can see that dividing the "Added" stat by the Leverage Index results in the exact numbers it should. I'm not going to try to explain why it works; I've tried that before. I'm hoping you can see that the math, which works perfectly in the no-hitting example, will work just as well within the framework of winning games.
WPA/LI is available at both Fangraphs and Baseball Reference.
Dave was called a "national treasure" by Rob Neyer. Seriously. Comments about this article can be sent to him through the miracle of e-mail.