Using Sabermetrics for Fantasy Baseball: Plate Discipline

No matter how high a particular player's BABIP may be, his average will be mediocre at best if he strikes out too much. This is why fantasy managers have known for years that players like Chris Davis are potential drains on a fantasy team's batting average. Furthermore, players that whiff a lot tend to continue to do so - it is a very sticky trait.

In 2019, the league average K% was 23%, meaning that roughly one in five MLB PAs ended in a K. That rate was virtually unchanged in 2020 with a 23.4 K%. Players who strikeout less frequently tend to hit for higher averages, while more strikeouts hurt batting average. Of course, a player may put up a fluke K% just as easily as a fluke BABIP.

Let's learn how analyzing stats related to plate discipline can help improve the performance of your fantasy baseball team entering the 2021 season.

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How to Interpret Plate Discipline

Sabermetrics may be used to determine whether a given player "deserved" his K% over a particular period. The first number to check is SwStr%. This metric simply tracks what percentage of pitches a batter swings and misses at. The league average was 11.2% in 2019 and 11.4% in 2020, with higher numbers indicating a proneness to K. If a player improves his strikeout rate without a corresponding improvement in SwStr%, the improvement is unlikely to stick moving forward. Likewise, a career-worst strikeout rate backed by a normal SwStr% is likely to regress in the player's favor.

Notably, Baseball Savant's Whiff% is not the same thing as SwStr%. Whiff% measures how often a batter swings and misses on all swings, while SwStr% uses all pitches seen instead. Whiff% figures are therefore much higher than SwStr%. Since SwStr% is used much more frequently in fantasy analysis, the rest of this article will use it.

Further detail is offered by O-Swing%, a measure of how often a batter swings at a pitch outside of the strike zone. Batters usually want to hit "their pitch," which they never get to see if they pop-up a fastball over their head early in the count. In 2019, the league averaged an O-Swing% of 31.6%. That fell by exactly one percentage point in 2020, to 30.6%. Numbers significantly higher than this indicate an increased likelihood of chasing a bad pitch and making poor contact or striking out.

This stat is also used to examine a player's walk rate, or BB%, in much the same manner as SwStr% is used to double-check K%. A strong walk rate when a player is still chasing too many pitches is not based on any repeatable skill, and will likely be normalized moving forward. Likewise, a lower walk rate paired with a career average O-Swing% indicates that the walks should come back.

Fantasy owners should always care about walks even if their format does not directly reward them. Every BB is a chance to steal a base or score a run, and players that know the zone tend to hit for higher averages to boot!

Evaluating Players Through Plate Discipline

Joey Votto is widely regarded as the master of plate discipline, and his surface stats support the assessment. His 16.6 BB% was really strong despite a down year overall, while his 19.3 K% bested the league average. Digging deeper, we find that these numbers are completely justifiable. His 19.5 O-Swing% was more than 10 percentage points better than the league average rate, and his 7.2 SwStr% was below the average as well. It is safe to conclude that Votto will continue to demonstrate outstanding plate discipline in 2021.

Cincinnati's Aristides Aquino does not measure up as well. He hit a reasonable .259 in 2019 despite striking out at a 26.7% clip. He chased too many pitches outside of the strike zone (41.2%), making it tough to project a repeat of his 7.1 BB%. Worst of all, he whiffed at 18.9% of the pitches he swung at, one of the worst marks in baseball. Fantasy managers drafted Aquino for elite power upside, but his plate approach was so bad that he quickly became unrosterable for both fantasy gamers and the Reds. He finished with a 2020 slash line of .170/.304/.319 with a 32.1 K% and 16.4 SwStr% over 56 PA.

Aggression or passivity at the plate can confound this analysis slightly. For example, Aquino was extremely aggressive in 2019 with a Swing% of 55.7%. The league average was 47% in 2019. Even if a hitter has a high chase rate, he can't strike out if he resolves the PA before three pitches are thrown. Aquino attempted to change his approach in 2020, dropping his Swing% to 47.3% against a league average of 45.9%. Votto is on the opposite side of the spectrum, as his refusal to swing at borderline pitches (36.2 Swing% last year) leads to more Ks than his raw SwStr% numbers would suggest.

Other plate discipline metrics exist, such as Z-Swing%, O-Contact%, and Z-Contact%, but SwStr% is usually a good enough proxy for fantasy purposes. One exception to this rule is a change in SwStr% rooted exclusively in pitches outside of the zone. Sometimes, missing those pitches can be better than hitting them.

Conclusion

To conclude, both K% and BB% are useful for fantasy purposes but fail to tell the whole story. SwStr%, or how often a batter swings and misses, is a better indicator of a player's future strikeout rate than K% alone. O-Swing%, or how often a batter chases pitches outside of the zone, performs similarly concerning BB%. If you would like to learn how to use more metrics to determine fantasy performance, check out our other articles on the subject!