While FIP is a useful tool to predict a pitcher's future ERA performance, fantasy owners should remember that ERA, not FIP, is what really matters in most formats. This means that we are interested in the "luck" that separates the two statistics.

While some of this luck is unpredictable, we can and should predict some of what goes into a pitcher's bottom line. BABIP plays a big role in the variation of a pitcher's perceived luck, but it may not be as clear-cut as it seems.

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How to Interpret BABIP for Pitchers

When calculating BABIP for hitters, we assume a neutral defense because they figure to see a balance of poor and skilled defenders as they travel around the league. This is not true for pitchers, as they always pitch in front of their own club's defenders. A team with Andrelton Simmons and his 32 Defensive Runs Saved (DRS) figures to provide better defense to its pitchers than a team that lacks a platinum glover. DRS is a counting stat like HR or RBI that measures how strong a defender a particular player is, with zero corresponding to an average defender and negative numbers possible for weak gloves. A better defense helps pitchers outperform their FIP.

Statcast makes it even easier to look at the quality of a pitcher's outfield defense. Outs Above Average, or OAA, measures each outfielder's defensive contributions using Catch Probability. If a batted ball is caught by an outfielder, the outfielder receives OAA credit equal to 1 - the ball's Catch Probability. For example, a successful catch on a ball with a 40% Catch Probability is worth 0.6 OAA (1 - 0.4 = 0.6).

Outfielders also lose points equal to the batted ball's Catch Probability if they flub the catch. Missing the ball in the example above would therefore subtract 0.4 from the player's OAA. Byron Buxton of the Minnesota Twins led baseball with 25 OAA last season, making Minnesota the best defensive outfield in baseball (31 total OAA).

BABIP is also partially determined by a pitcher's style. An extreme ground ball pitcher may have a higher BABIP against because grounders have higher BABIPs than fly balls (.241 to .130 in 2017.) This stylistic difference also changes how much a given pitcher will benefit from (or be hindered by) a particular defender on his team. For instance, a fly ball pitcher would love to pitch in front of Buxton, while a ground ball specialist would benefit more from an elite infielder like Simmons instead.

While defense is largely out of a pitcher's control, some pitchers can control their BABIP to a degree. For example, Yusmeiro Petit's .267 BABIP against last year feels way too low, but he can might sustain it. First, his 48.9% FB% means that many balls put into play against him figure to post low BABIP numbers. Second, he induced pop-ups at an impressive 18.6% IFFB% last season. Pop-ups are almost never hits, so inducing them consistently enables a pitcher to post better than average BABIPs.

The same principle holds for pitchers who can limit line drives, but this skill is not quite as sticky as pop-ups. Liners post very high BABIPs but randomly fluctuate, as we have seen in a previous article.

Finally, a pitcher's own defense may allow him to control the BABIP he allows. For example, Dallas Keuchel is an extreme ground ball pitcher (66.8% GB% last year) who nevertheless limited the BABIP against him to .256 in 2017. The reason why is the incredible nine DRS he compiled in his 145 1/3 defensive innings. Simmons had 1,369 2/3 innings to post his DRS last year, so extrapolating Keuchel's performance to Simmons's workload yields roughly 85 DRS!

Extrapolations like that aren't necessarily valid, so Keuchel is probably not the equal of two-and-a-half Andrelton Simmonses with a glove in his hand. Still, it seems insane to discount Keuchel's defense with the track record he has put together (42 career DRS in 984 2/3 innings).

Every pitcher allows a few hits, and the sequencing of these events may also cause a difference between a pitcher's FIP and ERA. Allowing three base hits over three innings is probably harmless, while allowing three hits in one inning and then nothing in the next two frames likely puts a run on the board.

Sequencing luck is measured by strand rate, or LOB%, and research shows that it is largely an unstable, luck-driven stat. In 2017, the league average LOB% was 72.6%, with higher numbers generally forecasting a higher ERA moving forward. Elite strikeout guys tend to be the best at getting the K "when they need it," and as such may sustain slightly elevated strand rates.

Conclusion

To conclude, a pitcher's BABIP includes some unknown variables but also some predictable inputs. The quality of his defense can help or hurt him. Sequencing does not affect BABIP necessarily, but can impact a pitcher's ERA substantially. A given pitcher's style, as a ground ball or fly ball specialist, may also impact his performance. Ballparks can have a substantial impact on a pitcher's ERA as well. We'll take a closer look at ballpark factors next time.

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