Plate Discipline: Using Sabermetrics for Fantasy Baseball

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 Mike Zunino 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 2021, the league average K% was 23.2%, meaning that roughly one in five MLB PAs ended in a K. Players who strike out 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.

FanGraphs is once again the best place to look for plate discipline data, located toward the bottom of each player's page right before fielding. Let's learn how analyzing stats related to plate discipline can help improve the performance of your fantasy baseball team entering the 2022 season.

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 11.3% in 2021, 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 2021, the league averaged an O-Swing% of 31.3%. 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 managers 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

Juan Soto has convincingly dethroned Joey Votto as the undisputed plate discipline king, and his metrics are a marvel to look at. His 22.2 BB% bested his 14.2 K% by a substantial margin, something virtually unheard of in today's game. Digging deeper, we see that Soto has the peripherals to back up those numbers. He only chased pitches outside of the strike zone 15.1% of the time last year, less than half of the league-average rate. He also rarely missed the ball with a SwStr% of 6.1. It is safe to conclude that Soto will continue to demonstrate outstanding plate discipline in 2022 and beyond.

Detroit's Javier Baez does not measure up as well. He hit a reasonable .265 in 2021 despite striking out at a 33.6% clip. He chased too many pitches outside of the strike zone (career-worst 46.6%), making it tough to project a repeat of even his modest 5.1 BB%. Worse, he whiffed at a career-worst 21.4% of the pitches he swung at. Baez has never been good at plate discipline, but it is still troubling to see everything trending in the wrong direction. The Tigers ignored these red flags by signing Baez to a big free agent deal in the offseason, but fantasy managers should not repeat their mistake.

Aggression or passivity at the plate can confound this analysis slightly. For example, Baez was extremely aggressive in 2021 with a Swing% of 56.8. Even if a hitter has a high chase rate, he can't strike out if he resolves the PA before three pitches are thrown. Soto is on the opposite side of the spectrum, as his refusal to swing at borderline pitches (35 Swing% last year) leads to more Ks than his raw SwStr% numbers would suggest. Still, you'd much rather see Soto's approach than Baez's.

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, stay tuned to our series on making sabermetrics more accessible to fantasy managers!