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Don't FIP Me Off

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I actually saw a comment on this at Fangraphs and it really amazed me how flawed of a statistic FIP is. I didn't think this got enough attention on Fangraphs, and I'm not sure if people realize how seriously flawed FIP may be here on the 'Pit, so I decided to write a post about it.

One of the biggest problems with FIP lies in the fact that it's based on innings pitched, as opposed to batters faced. Let's have a thought experiment:

Imagine two pitchers. Let's imagine them as genetically enhanced Dan Haren, because they never ever walk anybody. The only possible outcomes when they face batters, are hit, strikeout, or some other out. Now let's imagine that one of these pitchers has a BABIP of .400, whereas the other pitcher has a BABIP of .100. The following table illustrates what happens to these pitchers for every ten batters they face:

 

Pitcher A

Pitcher B

BABIP

.400

.100

Batters Faced

10

10

Strikeout Percentage

30%

30%

Strikeouts

3

3

Hits

4

1

Outs Recorded

6

9

Innings Pitched

2

3

K/9

13.5

9


K/9 is a basically an input into FIP. As you can see, FIP benefits greatly from a high BABIP.

I see two major implications to this. First, to the extent that BABIP is luck, then what this means is that when we analyze a pitcher's FIP and say that he's gotten unlucky with BABIP and is due for regression in the future, he is not going to regress all the way down to his current FIP. Yes, his ERA will regress, but it's probably not going to regress as much as his FIP would suggest.

The second implication is that to the extent BABIP is not luck, FIP underrates pitchers who have lower BABIPs. In other words, flyball pitchers and pitchers that pitch in pitching parks are underrated by FIP.

So yeah, I guess the next time we see FIP stats and proclaim that regression is about to happen, it's definitely worthwhile to take a closer look at what exactly is going on in the background.