xFIP
Expected Fielding Independent Pitching (xFIP) is a regressed version of FIP, developed by Dave Studeman from The Hardball Times. It’s calculated in the same way as FIP, except it replaces a pitcher’s home run total with an estimate of how many home runs they should have allowed. This estimate is calculated by taking the league-average home run to fly ball rate (~9-10% depending on the year) and multiplying it by a pitcher’s fly ball rate.
Home run rates are generally unstable over time and fluctuate around league-average, so by estimating a pitcher’s home run total, xFIP attempts to isolate a player’s ability level. A pitcher may allow home runs on 12% of their flyballs one year, then turn around and only allow 7% the next year. HR/FB ratios can be very difficult to predict, so xFIP attempts to correct for that.
Here is the full formula for xFIP. Notice how it is almost exactly the same as the formula for FIP, with the lone difference being how each accounts for home runs:
xFIP = ((13*(FB% * League-average HR/FB rate))+(3*(BB+HBP-IBB))-(2*K))/IP + constant
The constant is solely to bring FIP onto an ERA scale and is generally around 3.20. You can find historical FIP constant values in this spreadsheet, or you can derive the constant by taking league-average FIP and subtracting that from league-average ERA. League-average home run per fly ball rate varies on a yearly basis, but you can find those values here on the FanGraphs leaderboards.
Along with FIP, xFIP is one of the best metrics at predicting a pitcher’s future performance. Since it was created, though, there have been some studies that suggest certain pitchers can post lower-than-average HR/FB rates over time. For more information on this, see the statistic SIERA.
Context:
Please note that the following chart is meant as an estimate, and that league-average xFIP varies on a year-by-year basis so that it is always the same as league-average ERA. To see the league-average xFIP for every year from 1901 to the present, check the FanGraphs leaderboards.
| Rating | xFIP |
|---|---|
| Excellent | 2.90 |
| Great | 3.25 |
| Above Average | 3.75 |
| Average | 4.00 |
| Below Average | 4.20 |
| Poor | 4.50 |
| Awful | 5.00 |
Things to Remember:
● While HR/FB ratios are generally unstable over time, some pitchers are still more prone to allowing home runs than others. If a pitcher has a long history of over- or under-performing the league average with their HR/FB rate, then you can reasonably expect them to perform closer to their career average than the league-average. In cases like this, xFIP may overestimate or underestimate a player’s true talent level by assuming a league average HR/FB ratio. Again, for more, see SIERA.
● Ground ball pitchers typically have higher HR/FB ratios than fly ball pitchers.
● xFIP has one of the highest correlations with future ERA of all the pitching metrics. Only SIERA out-paces it.
Links for Further Reading:
Batty for Baseball Stats – Hardball Times
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Quick note on this:
“- Similarly, groundball pitchers typically have a higher homerun rate than flyball pitchers.”
I understand what you’re saying there — that groundball pitchers have higher HR/FB rates than flyball pitchers (due I think to the fact that their flyballs are more often the result of bad pitches). But the term “homerun rate” is potentially ambiguous if read out of context, as someone might think you were saying that groundball pitchers give up more home runs (per nine innings) than flyball pitchers do.
P.S. This dictionary idea is awesome! I especially like the percentile tables.
Great call, I’ll clarify. I feel like “homerun rate” looks less weird to people than HR/FB rate, but it can be confusing especially in this instance. Many thanks.
Wait, doesn’t xFIP taken into account flyball percentage? Thus a pitcher with a 5 K/9, 3 BB/9, and a 48 FB% will have a higher xFIP than a pitcher with a 2 K/9, 3 BB/9, and a 40 FB%
Nope, it only considers walks, strikeouts, and HR/FB rate….and the HR/FB rate is regressed to league-average for all players.
tERA and SIERA (BPro) are the two pitching stats that include batted ball data.
According to the calculator link, what Jon says is true. FB%*.106 gets you the expected HR portion.
Could you add the formula to this post (I see it on FIP, but not xFIP)?
Ahh bugger, yeah, right. I saw Dave Cameron address it in the chat this morning, and you’re right…FB% is included to help you get the expected HR portion. So Jon, you were basically right…FB% is included and helps get the expected HR portion of xFIP.
I would hesitate to say that Jon’s above example is correct without running it through the calculator, though…it all depends on the relative weights of each section. But yeah, sorry Jon…good catch all around.
Yeah I asked Dave about it in the chat and got clarification that my original thought of it was correct. And no problem, I’m just happy I have a clearer definition of xFIP now.
I messed this example up by the way, I meant to give the pitchers identical K and BB rates, ha.
Ah ok, thanks for the clarification! Yikes, I can’t believe I had it wrong the whole time.
Why can’t I type anything in the cells in the calculator?
I have been reviewing pitchers statistics for 2010 – 2011. I notice that their xFIP for that period is different than just for 2011. For instance, Jered Weaver has an xFIP of 3.42 for 2011 and 3.35 for the period 2010-11. Which is the one I should use when comparing him to other pitchers ? Is the 2011 a more current expectation and thus the better indicator ?
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