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Last week, we published an article on minor league plate discipline metrics for MLB prospect hitters, covering contact %, swinging strike %, swing %, foul strike %, called strike %, and ball %.
This article today looks at the same set of metrics but for pitchers. The full minor league plate discipline leaderboards will continue to be updated monthly (they can be found here, at Scout the Stat Line).
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What insight can we glean from plate discipline metrics for pitchers?
While BB% and K% themselves are sticky, reliable metrics for pitchers that are indicative of talent in relatively small samples, plate discipline metrics like whiff % are even more reliable, and thus particularly useful in smaller samples.
Further, comparing a pitcher’s BB% and K% with their plate discipline metrics can isolate outliers who deserve a better BB% or K% given their plate discipline metrics. For instance, pitchers with a high whiff % tend to also have a strong K% minus BB%: across the minor leagues (minimum 500 pitches), whiff %, and K% minus BB% are strongly correlated at .71.
Finally, K% minus BB% is a key single metric of pitching talent that at least holds its own when faced up against more complicated metrics with good predictive power like SIERA or xFIP.
Expected K% minus expected BB%
To analyze whether a pitcher’s K% minus BB% is consistent with their “deserved,” or expected K% minus BB% given their plate discipline metrics, I produce an expected K% minus BB% metric that shows the expected value of a pitcher’s K% and BB% given their whiff % (whiffs divided by swings), called strike % (called strikes divided by total pitches), foul strike % (foul strikes divided by total pitches), and swing % (swings divided by total pitches).
Each of these four plate discipline metrics is strongly associated with a higher K% minus BB%. Together, they explain 84% of the variance (r-squared) in K% minus BB% (n = 1552 players in 2022 across the minor leagues with a minimum of 750 TBF). If a pitcher has a better K% minus BB% than expected K% minus BB%, they are benefiting from some combination of luck (in terms of sequencing, for example) and skill not captured by these four metrics (in the future, I will bring in location to differentiate between pitches in the zone and out of the zone).
Looking at the data on some noteworthy pitching prospects
The table below shows plate discipline metrics and expected K% and BB% for a group of pitching prospects (sorted alphabetically) that has made a lot of noise this year both in terms of performance and scouting buzz.
The Marlins’ Eury Perez has one of the top expected K% minus BB%s at Double-A, despite being one of the youngest pitchers at the level. His strong expected K% minus BB% is driven by an elite 37% whiff % (my whiff % follows Baseball Savant, including foul tips because they are similar enough to whiffs), well above the 29% minor league average. Perez also fills the strike zone: his ball % is very low at 30%, eight percentage points below the minor league average.
Toronto’s Ricky Tiedemann has been soaring through industry prospect rankings as perhaps this year’s biggest breakout pitching prospect. His plate discipline metrics very much validate the hype: he broke Class A with a 45 whiff %, and he hasn’t slowed down much at High-A with his 35 whiff %. His 34% xK% minus xBB% at Class A is among the top-five strongest in the minors this year, and his 25% xK% minus xBB% at High-A is almost as elite.
Conclusion
Readers are already accustomed to accounting for plate discipline metrics when analyzing major league pitchers, looking at things like whiff % to measure pitcher talent and better understand how sustainable a pitcher’s K% and BB% are.
The same data is available at the minor league level, but it has for a long time not been easily accessible: it requires some sort of statistical analysis software to access. With the new minor league plate discipline leaderboards featured in this article, readers can now dig deeper when analyzing minor league talent.
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