In looking at BABIP, we divided at-bats into two categories: times when the ball was put into play and times when it was not. Whether balls in play will result as a hit or an out is largely out of the hands of the pitcher. With this in mind, multiple stats have been developed that focus on at-bats when the ball is not hit into play.
The most well-known of these stats is Fielding Independent Pitching (FIP). Which looks only at walks, batters hit by a pitch, strikeouts, home runs allowed, and innings pitched—in other words, things that are much more within the pitcher’s control than balls hit into play.
FIP = ((3(BB + HBP) + 13HR – 2K) / IP) + constant*
As described by Fangraphs, FIP is used to “to calculate what a player’s ERA should have been over a given time period.” By ignoring balls in play it is assumed that factors like defense is equal for all pitchers.
*The extra number added on at the end, which is typically about 3.1, puts FIP on approximately the same scale as ERA, which makes it easier to recognize a good one from a bad one. It changes every year based on a number of factors.
By ignoring so many at-bats one would think FIP only gives us part of the picture of a pitcher, whereas ERA or WHIP take into account all batters a pitcher faces. However, what we find is that FIP is actually a better indicator of the degree of success a pitcher has in subsequent seasons than ERA. This is likely due to walk- and strikeout-rates being two of the most consistent stats for pitchers from year-to-year.
A strikeout or a walk in one stadium is exactly the same in any other, but a home run at Fenway Park may not be one at Safeco Field. This makes home runs a bit tricky to deal with when we are trying to limit the number of factors other than the pitcher himself in FIP.
On average 10% of fly balls will end up as home runs. Some pitchers may end up finishing the season with 8% of their fly balls having left the yard, while others will be unlucky and have 14% of theirs flying over the fence.
In order to eliminate this element of luck, we have a variation of FIP called xFIP. Rather than using the number of home runs a pitcher allows, xFIP assumes every pitcher has the average fly ball to home run ratio.
FIP = ((3(BB + HBP) + 13(Flyballs * League-average HR/FB rate) – 2K) / IP) + constant
Though the difference between FIP and xFIP will not necessarily be a lot, we find that xFIP is more predictive of subsequent performance than FIP.