The Updated xwFIP

Photo Credit: Daniel Varnado

I received a very good comment from reader Light_Saberist on one of my latests posts, in which I compared the predictive powers of xwFIP and FIP. In order to determine the xHR of a pitcher, xFIP (not to be confused with xwFIP) uses the league average homerun/flyball rate of 10.5% and multiplies that rate by the number of flyballs the pitcher allows. This logic may mitigate some of the luck factors in allowing homeruns and thus strengthen xwFIP as well.

I checked that hypothesis, and it was correct!

Linear regression between the updated xwFIP, using xHR instead of HR, and Next Year Era is seen in Figure 1 below. Moreover, Table 1 lists the correlations between each pitching metric and Next Season ERA. Combining Figure 1 and Table 1, we can see that the improvement in predictive power from FIP to xwFIP is now greater than the improvement from ERA to FIP.

Figure 1: Next Year ERA (y-axis) vs. xwFIP (x-axis), using xHR, for the 1466 pitchers with at least 50 innings pitched in back-to-back seasons since 2015

Table 1: Correlation between each pitching metric and Next Season ERA. Thanks to Light_Saberist for calculating the correlations for xFIP and xwOBA!

Now, lets take a look at 2023 pitching performances through the lens of the updated xwFIP.

Looking at Table 2, which lists the 25 pitchers who benefit the most from the use of xwFIP in 2023, we see that veterans Corey Kluber, Luis Severino, Johnny Cueto, and Carlos Rodon were quite unlucky in 2023. Based on the quality of contact they generated and using the league-average HR/FB rate in xwFIP, both Severino and Rodon can be expected to significantly reduce their ERAs in 2024. Kluber and Cueto are currently free agents.  

Table 2: The 25 pitchers who benefit the most from the use of xwFIP in 2023

On the other hand, relievers Chris Martin and Ryan Brasier are joined by starter Sonny Gray and closer Emmanuel Clase as the pitchers who were quite lucky in 2023, limiting damage despite hard contact and managing to keep more than their fair share of flyballs in the ballpark.

We see in Table 3 below, that their xwFIPs are more than a full point higher than their FIPs in 2023.

Table 3: The 25 pitchers who benefit the least from the use of xwFIP in 2023

Assessing the 25 pitchers with the lowest xwFIP in 2023, listed in Table 4 below, we see that Felix Bautista still reigns supreme among all pitchers with at least 50 innings pitched in 2023, boasting a minuscule xwFIP of 2.04.

Aroldis Chapman, Spencer Strider, AJ Puk, and Gabe Speier join Bautista in the top five of this list.  

Table 4: The 25 pitchers with the lowest xwFIP in 2023

When evaluating the 25 starting pitchers with the lowest xwFIP in 2023, we see that Spencer Strider, Tarik Skubal, Kevin Gausman, and Logan Webb are in the elite tier, with an xwFIP less than 3.00. Starters Tyler Glasnow, Zach Eflin, Freddy Peralta, Nick Pivetta, and Blake Snell also posted outstanding xwFIPs under 3.30.  

Table 5: The 25 starting pitchers with the lowest xwFIP in 2023