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This article looks at plate discipline metrics for minor league hitters including contact % and swing % to help fantasy leaguers evaluate contact and patience skills among hitting prospects.
Swing % and contact % are among the quickest, if not the quickest, metrics to stabilize for hitters. ‘Stabilization’ is a term used to indicate a metric’s reliability or its correlation with itself in the future. More stable metrics provide a more meaningful indication of a player’s talent in smaller samples.
Looking at plate discipline metrics can give you an idea of whether a hitter’s K% or BB% is sustainable or not, especially in smaller samples.
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Gathering the Data
Though FanGraphs publishes a full suite of plate discipline metrics for major leaguers and certain metrics for minor leaguers (e.g., swinging strike %), much remains unpublished. Thanks to Bill Petti’s indispensable baseballr package for r, this data is accessible to the public, although it demands a fairly slow and painstaking collection process if you’re pulling a lot of data (Petti outlines the process here).
For this article, I have compiled a leaderboard capturing minor league data for all hitters Class A or above, with a minimum of 100 pitches seen, through Saturday’s (6/25) games. The leaderboard features:
- contact % (the percentage of total swings a hitter makes contact on, including fouls but excluding foul tips)
- swing %
- called strike %
- foul strike %
- swinging strike % (including foul tips)
- ball %
For swing %, called strike %, foul strike %, swinging strike %, and ball %, the denominator is total pitches, whereas for contact % it's total swings).
You can view the full leaderboards at Scout the Statline, a website I co-publish with Ross Jenson as a house for our various metrics and leaderboards.
I leverage past research from Mike Podhorzer and Jeff Zimmerman to develop expected K% and BB% metrics for each hitter. These metrics show a hitter’s expected K% and BB% given his contact %, swing %, called strike %, and foul strike %. Zimmerman's MLB models also capture whether a pitch was in the zone or outside the zone, but this data is not easily reproducible at the minor league level – an area for future research. I regress 2022 K% and 2022 BB% minus K% on the 2022 plate discipline metrics, a form of descriptive regression following past norms in expected hitter metrics.
Together, the four metrics explain 92% of the variance in hitter K% and 86% of the variance in hitter BB% minus K% (I train the model on n = 351 Class A Advanced hitters). I then use expected BB% minus K% and expected K% to derive an expected BB% (note: hitter BB% minus K% is a superior metric for hitters, capturing the combination of patience and contact skills, in the same way that K% minus BB% is a superior metric for pitchers). A higher contact % is associated with a lower K% and a higher (better) BB% minus K%, whereas called strikes and foul strikes are associated with worse K% and BB% minus K% outcomes. A higher Swing % is associated with a lower K%, but also a lower (worse) BB% minus K%.
Expected K% and BB% for Notable Prospects
Table note: minor league averages: 71% contact, 47% swing, 16% called strike, 17% foul strike, 14% swinging strike, 37% ball, 16% in-play strike. Averages only vary slightly from league to league and level to level.
The above table shows expected K% and BB% risers and fallers for some noteworthy top prospects. Expected K% and BB% paint a brighter picture for many top prospects, particularly Jarred Kelenic, Nolan Gorman, and Jo Adell. Adell’s 70% contact suggests he deserves better than the 33% Triple-A K% he has posted so far, and the model foresees a stronger xBB% as well. Most of his plate discipline metrics are close to Triple-A average, yet his BB% minus K% is below average – I’d bet on some gains moving forward.
I included Gunnar Henderson and Esteury Ruiz here because they are two of the biggest year-over-year improvers in terms of BB% and K%, and I was curious how “real” their improvements were. Expected BB% and K% fully validate Ruiz’s performance thus far. Henderson may deserve a slightly worse K% and BB% according to the expected metrics, but he has been so outrageously good this year that his xK% and xBB% are still elite even allowing for some decline. Similarly, many of the decliners featured above, like Royce Lewis, and James Wood, have been good enough by K% and BB% to still look good even allowing for some decline. On the other hand, Vidal Brujan’s declines are a bit worrisome, especially when paired with his elevated K% at the MLB level.
In addition to the names in the table above, a few additional names are worth mentioning on account of how good they have been in blending patience and contact abilities: Adael Amador and Jorbit Vivas. Both are among the minor league leaders in xBB% minus xK%, validating their exceptional BB% minus K% (Amador's xBB% is 13% and his xK% is 11%, while Vivas' xBB% is 13% and his xK% is 9%). Both Amador and Vivas are among the youngest players at their level, Class A and Class A Advanced, respectively, and both have shown some pop, unlike many of the other league leaders in xBB% minus xK%. Both guys have gotten a fair amount of love in prospect circles, but I'd argue they still deserve even more – both remain underappreciated.
Conclusion
This article dug deep into plate discipline metrics to better evaluate contact and patience skills for minor leaguers. I plan on producing a similar expected K% and BB% leaderboard for minor league pitchers shortly. I will also update the minor league hitters leaderboard hopefully monthly (it’s a pain or else I’d do it more frequently). When analyzing hitter patience and contact skills, don't stop at BB% and K%. Diving into plate discipline metrics can help you determine how sustainable a given BB% or K% is.
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