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*(Flyballs * League-average HR/FB rate))+(3*(BB+HBP))-(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 here, 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.
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.
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.
Links for Further Reading: