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, 13.7% 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, Giancarlo Stanton is generally regarded as one of the top sluggers in the game today. His HR/FB was 34.3% in 2017, nearly triple the league average rate. If this number regressed to the league average, Stanton wouldn't be very good. However, he has a career rate of 26.9%. Clearly, above average power is something Stanton just does. Last year was special, but 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. Elite sluggers generally post a fly ball percentage of around 40%. Subjected to this test, Stanton had a 39.4% rate in 2017 and a career mark of 40.4%. These rate stats, combined with a consistently above average HR/FB, make Stanton the player he is.

Stanton doesn't really illustrate the distinction between HR/FB and FB% because he excels at both. For a predictive illustration, consider his former teammate Christian Yelich. His HR/FB last season was a strong 15.3%, suggesting that he should have hit a few bombs for fantasy owners. Yet he managed only 18 big flies in 695 PAs last season. The reason is a tiny 25.2% fly ball rate, a rate too low to do anything with even Stanton's power.

Yelich managed 21 big flies in 2016, but his underlying 23.6% HR/FB was considerably higher than his career 16.2% rate. He hit very few balls into the air that year (20% FB%), meaning that any loss of HR/FB would cripple his fantasy value. Yelich wasn't a complete bust thanks to an uptick in FB% last season, but he still failed to clear the 20 HR plateau in a year where seemingly everyone hit 25+.

Joe Mauer of the Minnesota Twins 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 today, ruining many 2010 fantasy seasons and saddling the Twins with one of the worst contracts in MLB.

If you're looking for the 2018 version of 2010 Mauer, Eric Hosmer (22.5% HR/FB, 22.2% FB%), Tommy Pham (26.1% FB%, 26.7% HR/FB), and Domingo Santana (27.7% FB%, 30.9% HR/FB) all seem like strong candidates for power regression.

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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