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 owners have known for years that players like Adam Dunn, Chris Carter, and 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 2016, the league average K rate was 21.1%, meaning that roughly one in five MLB PAs ends in a whiff. Players that K significantly less than this have an advantage in hitting for a higher average. Players that whiff more often tend to post lower averages. This is relatively common knowledge for most fantasy owners.
Editor's note: Be sure to check out all our strategy articles on how to win your fantasy baseball leagues: Our Sabermetrics series - Part 1: BABIP for Hitters, Part 2: HR/FB%, Part 3: Batted Ball Distribution; Points League Primer; Using SIERA to Win Your League; How Your Brain Messes with Your Drafts; and Why You Shouldn't Overpay for Saves.
How to Interpret Plate Discipline
Sabermetrics may be used to determine whether a given player "deserved" his K rate over a particular period, avoiding misleading data the same way BABIP is used to see through a fluky average. The first number to check is SwStr%, alternatively called whiff rate. This metric simply tracks what percentage of a batter's swings fail to make contact with a pitch. The league average is around 10 percent, with higher numbers indicating a proneness to K.
SwStr% tends to increase if a batter swings harder, making power hitters more susceptible to the strikeout than other players. 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.
Further detail is offered by O-Swing%, a measure of how often a batter swings at a pitch outside of the strike zone. Generally, swinging at pitches outside of the zone is a bad idea. 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 2016, the league averaged an O-Swing% of 30.3 percent. 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 in 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!
Let's look at some examples of advanced plate discipline stats in action. Joey Votto is widely regarded as the master of plate discipline, and his surface stats support the assessment. His 16 percent BB% was nearly equal to his 17.7 percent K rate, after all. Digging deeper, we find that these numbers are completely justifiable. His 20.8 percent O-Swing% was nearly 10 percentage points better than the league average rate, and his 7.1 percent SwStr% was better than average average as well. It is safe to conclude that Votto will continue to demonstrate outstanding plate discipline in 2017.
Baltimore's Jonathan Schoop, however, may not be so lucky. His 21.2 percent K% and resulting .267 batting average were fine in 2016, fooling many preseason prognosticators who felt his swing and miss profile would doom him to a terrible batting average. A horrific 43 percent O-Swing% and 16.2 percent SwStr% are hiding under the acceptable surface stats though, threatening to ruin any fantasy owner who does not prepare for the batting average risk Schoop continues to represent.
Aggression or passivity at the plate can confound the analysis slightly. For example, Schoop's teammate Adam Jones regularly outperforms his swinging strike rate by avoiding two-strike counts. Even if a hitter has a high whiff rate, he can't strike out if he resolves the PA before three pitches are thrown. Joey Votto is on the opposite side of the spectrum, as his refusal to swing at bad pitches puts him in more two-strike counts leading to more Ks than his SwStr% numbers would suggest.
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 rate alone. O-Swing%, or how often a batter chases pitches outside of the zone, performs similarly concerning BB rate. 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 if an older player sees a decline in Z-Contact%, indicating that he can no longer make contact on pitches he used to hit in the zone. Another is a rise in SwStr% rooted exclusively in pitches outside of the zone. Sometimes, missing those pitches is better than hitting them.