Using BABIP to predict a player's batting average is great. Average is a category in many league formats, and every hit is an opportunity to steal a base or score a run. But most owners find the long ball sexier.

Every HR comes with a guaranteed run scored and at least one RBI. Many owners build their teams around power for this reason. Yet fluky HR campaigns can happen just as easily as fluky batting average ones.

How do we tell the difference between a legitimate breakout and a fluke?

How to Interpret HR/FB

HR/FB measures the percentage of fly balls that leave the park. Last year, 12.7% of all fly balls ended up in the seats. Like BABIP, an experienced player's personal benchmark in the stat is a better indicator of his future performance than the league average. For example, Khris Davis is generally regarded as one of the top sluggers in the game today. His HR/FB was 24.1% in 2018, nearly double the league-average rate. If this number regressed to the league average, Davis wouldn't be very good. However, he has a career rate of 23.6%. Clearly, above-average power is something Davis just does. He should continue to crush bombs with regularity.

Large spikes or dropoffs in HR/FB are generally temporary, meaning that the stat is usually not predictive of a power breakout. Fantasy owners want to know the next power breakout, so this may be somewhat disappointing. Future power production may be predicted, however, by an increase in fly ball rate, or the percentage of a batter's flies as opposed to liners or grounders. There are limits here, as Billy Hamilton is never helping a fantasy team with his power no matter how many fly balls he hits. Still, FB% is generally the stat you want to look at for power potential.

Elite sluggers generally post a fly ball percentage of around 40%. Subjected to this test, Davis had a 48.8% fly ball rate in 2018 and a career mark of 42.2%. These rate stats, combined with a consistently above average HR/FB, make Davis the fantasy player he is.

Davis doesn't really illustrate the distinction between HR/FB and FB% because he excels at both. For a predictive illustration, consider 2018 NL MVP Christian Yelich. His HR/FB last season was an unbelievable 35%, powering fantasy rosters with a total of 36 long balls. Imagine how many long balls he could have hit if his FB% was higher than 23.5%.

Unfortunately, it seems unlikely that Yelich will be able to replicate his 2018 success in the power department. His career HR/FB is only 20.3% including his monster campaign last season, suggesting that significant regression is in order. Meanwhile, his career FB% (20%) is actually slightly lower than his 2018 rate. A dramatic swing change to join the proverbial "fly ball revolution" seems especially unlikely coming off an MVP season, setting Yelich up as a potential fantasy bust in 2019.

The recently retired Joe Mauer provides the best illustration of trusting HR/FB without regard for FB%. In 2009, Mauer went bonkers with a .365/.444/.587 line and 28 bombs. His HR/FB% spiked to 20.4% that season, but nothing in his history indicated he could maintain a level that high as his previous career best was 10.8% in 2006. Meanwhile, his 29.5 FB% was far too low to expect any real power production moving forward. He received first-round attention from fantasy owners in 2010, and the Twins gave him an extension they could not really afford. Mauer morphed back into the singles hitter we know him as, ruining many 2010 fantasy seasons and saddling the Twins with one of the worst contracts in MLB.

If you're looking for the 2019 version of 2010 Mauer, Eric Hosmer (19.4% HR/FB, 19.7% FB%), Shohei Ohtani (29.7% HR/FB, 32.9% FB%), and Ian Desmond (24.7% HR/FB, 21.5% FB%) all seem like strong candidates for power regression (in addition to Yelich). All three of the players listed here last year (Hosmer, Tommy Pham, and Domingo Santana) disappointed fantasy owners who drafted them in 2018, so consider yourself warned.

Conclusion

HR/FB is considered the BABIP of power because it can be used to evaluate whether a given player is outperforming his true talent level. A player with a large spike or decline in HR/FB should generally be expected to return to his established baseline moving forward. Ballpark factors may alter HR/FB, but in general raw fly ball percentage is a better tool to identify potential power breakouts.

Of course, it is possible for a batter to legitimately change his approach and permanently boost his HR/FB. Statcast allows us to measure precisely how hard a player is hitting the ball, potentially validating a performance that would otherwise be labeled a fluke. We'll take a closer look in Part 3!

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