Plate discipline is an area of a hitter's game that stat-oriented fans covet as much as traditional thinkers. The more a player walks, the more he gets on base, and the less he strikes out, the more chances he has to get on base. It's not as important to hitters as it is to pitchers, but it is one of the first attributes that stabilizes with a hitter.
I decided to look for players whose plate discipline stats did not match his BB rate. For BB%, I used (BB-IBB)/(PA-IBB). For the dependent variables, I used Swing%, Contact%, and Zone%, doing two different models for BIS and PFX data. With a sample including all players with at least 100 PA last year (459 players), each set of data resulted in an R-squared of 54%.
The range of Actual-Expected differences were +6%, with 88% of the hitters within 3% on either side. On the negative side, Brooks Conrad and Brett Hayes each walked about 6% less than expected. Of all the qualifying hitters, only Corey Hart and J.J. Hardy walked over 3% less than expected. These two players are an example of the varying types of players found on the extreme ends of the list, a good sign for an unbiased model.
On the other end, Daric Barton and Jason Giambi walked 6% more than expected in their limited time. Joey Votto and Adam Dunn were the two regulars who walked at least 4% more than expected. Those two aren't much different aside from Dunn's contact "issues." Right below them are Miguel Montero and Carlos Santana, two different-style hitters.
If you're wondering why I used such a low minimum for PA, it's because I wanted to include Diamondbacks CF Adam Eaton in the analysis. With such a low threshold, the model is bound to be faulty at the extremes, but Eaton represents a paradox that seemingly can't be repeated.
He was patient during his September callup, swinging about 4% less than the average hitter. Based on the model, that added about 1.8% to his expected walk rate. However, he made contact about 9% more than average, taking off 1.1% of the walks. His Zone% figured varied greatly, even in respect to average, among the two data sets. The BIS data had 3.5% less pitches in the zone, adding about 0.8% to his walk rate, while PFX had just over 2% more pitches in the zone, subtracting about 0.7% from his walk rate.
When all was said and done, BIS data predicted a 9.1% BB rate, while PFX predicted a 7.8% rate. Eaton's actual BB rate was a much higher 13.6%. Using Statcorner's AAA plate discipline stats, Eaton had almost identical swing and contact rates in AAA last year, only posting a 9% BB rate. Before that, he did put up consistent double-digit walk rates, but we don't have any plate discipline stats to analyze.
With Eaton's speed and below-average, but not poor, power, I expect his Zone% to be more like the PFX data, above average. The projections at FanGraphs show the fans having a higher confidence he can continue to walk, one of the few times I'll actually defer to the systems.
While I'm trying to ease any thoughts of him getting a lot of free passes, he still looks to be a solid player. He's been hit by at least 20 pitches each of his three professional seasons, which makes up for some of the lost walks likely to come. His K rate is below average, able to use his good speed to get on base and wreak havoc. A average to above-average hitter who can play a decent CF and run the bases is a valuable piece to any team.