Analyzing xFIP for Regression Candidates

xFIP is an underlying metric that can be used as a predictive ERA estimator. By definition of Fangraphs: xFIP is a regressed version of Fielding Independent Pitching (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 given the number of fly balls they surrendered while assuming a league average home run to fly ball percentage (between 9 and 10% depending on the year).

In other words, it normalizes a pitcher’s home run rate while also using FIP to create an ERA estimate. There are some big issues with this metric and I see it being used wrong all of the time. Some pitchers are extremely home run prone while others aren’t and that is something xFIP seems to ignore. This is why using SIERA and K-BB% to predict ERA is by far better than xFIP.

Examples:

Here is Andrew Heaney’s ERA/xFIP by year:
2018: 4.15/3.68
2019: 4.91/4.18
2020: 4.46/4.15
2021: 5.83/4.12

Notice how the xFIP consistently calls for improvement but it never comes? That’s because Heaney has a career 1.62 HR/9 a very high rate. xFIP wants to normalize that rate, but why would we do that when Heaney has proven to give up a lot of home runs year in and year out?

Here is Justin Verlander’s ERA/xFIP by year:
2016: 3.04/3.78
2017: 3.36/4.17
2018: 2.25/3.03
2019: 2.58/3.18

xFIP consistently called for regression with Justin Verlander. Why? Because it tries to normalize his home run rate when his career rate is 0.93. Verlander has always suppressed home runs so why would we normalize it?

xFIP can be useful though, say a pitcher like Verlander (assume he is 28 years old though) suddenly goes from a career home run rate of 0.93 to 1.62 for one season. That is where you use xFIP and that is when it could be useful. Always be careful when using it or seeing people using it because you have to use it in the right circumstance.

Starting pitchers where xFIP is relevant from 2021:

Aaron Nola

This is the perfect example of when to use xFIP. Nola finished 2021 with a 4.63 ERA and 3.37 xFIP. Most importantly Nola’s career home run rate average is 1.2 and last season he sat at 1.3. That’s a big difference and seeing that his history shows his home run rate should dip we can look at that 3.37 xFIP and see it as his future ERA.

Kyle Hendricks

Hendricks had by far the worst season of his career in 2021 posting a 4.77 ERA in 181 innings pitched. A fun fact about Hendricks is that in the seven seasons prior to this year he never had an ERA over four. Basically, he has been the most consistent pitcher in recent memory.

His 2021 4.77 ERA came with a 4.61 xFIP. I wanted to point out Hendricks because his xFIP is showing slightly better days ahead if you normalize his home run rate. While it is a small amount I think if you look at Hendricks past you might see how much better he really can be in 2022. For his career, Hendricks has had a 0.99 HR/9 and in 2021 he sat at a 1.54 mark.

At age 32 could he maybe be on the downtrend? 32 isn’t that old for a pitcher anymore and pitchers like Hendricks usually fair better in aging compared to high-velocity pitchers.

Hyun-Jin Ryu

Ryu is an interesting case because there are a lot of different factors that come into play for his 2021 season. Overall, Ryu pitched 169 innings to the tune of a 4.37 ERA, the worst of his career.

There are two factors to take into account with Ryu. The first is the fact that he couldn’t see his family for the entire season. He has publicly spoken out about how much it affected his mental health and how playing in Buffalo, NY really limited his time with his family. The second one is the stadiums he played in Buffalo, they were dream parks for hitters making it very difficult for weak contact pitchers like Ryu.

Ryu’s 4.37 ERA came with a 3.94 xFIP showing improvement if Ryu can stabilize his home run rate. Ryu’s career HR/9 is an immaculate 0.94 and last season it rose to 1.28. Besides 2019 Ryu has never had an HR/9 over 1.00 and I think it’s a testament to how difficult it was for him to pitch in Buffalo. Luckily next season they will be in Toronto and I think you can easily look at Ryu’s xFIP of 3.94 and assume his ERA for next season will be sub-four.