The Maddux Plate Discipline Index (mPDI) for Hitters

Last week, I quantified a famous quote by Hall of Fame pitcher, Greg Maddux. He preached that the key to pitching is to throw a strike when the batter isn’t going to swing, and to throw a ball when the batter will [swing].

The introductory article to wPDI, the Weighted Plate Discipline Index for pitchers, can be found here.

Today, let’s turn the tables around and jump into the hitting equivalent. We can enumerate the offensive parallel of the quote – and evaluate what would be the “Maddux keys to hitting.” A Maddux hitter would swing at pitches when they are thrown in the zone and would lay off of pitches thrown out of the zone.

We will use the wPDI framework to help us quantify what we will call a “Maddux hitter.” For the hitting version of mPDI – the weights of the outcomes will be the exact inverses of the pitching indexes.

Again, here is a quick glossary of what the six possible plate discipline outcomes look like:

Maddux Plate Discipline Index (mPDI) for Hitters:

The formula for mPDI, the Maddux Plate Discipline Index:

mPDI = Index_A * A% + Index_B * B% + Index_C * C% + Index_D * D% + Index_E * E% + Index_F * F%

A%, B%, …, F% is defined exactly as in wPDI – the percentage of total pitches per each outcome.

The mPDI indexes for hitters are given as:

Since the indexes for C, D, E are all 100%, with A, B, F% all set to 0% – we can simplify the Maddux Plate Discipline Index (mPDI) for hitters as:

mPDI = C% + D% + E%

This represents the total percentage of outcomes where the batter swings at a pitch in the zone [Outcomes D & E] or looks at a pitch thrown outside of the zone [Outcome C]. We obviously know that hitting is far more complicated – but these are the outcomes for which hitters demonstrate plate discipline as alluded to by Greg Maddux. I will refer to outcomes C, D & E as the Maddux hitter outcomes [A, B & F – would be the Maddux pitcher outcomes].

mPDI for hitters tops out at just under .800 for the elite, and bottoms out just under .600. The average for mPDI for hitters is approximately .680.

Below are the 2018 leaderboards for mPDI (min 100 PA):

Joey Votto is the king! I can’t say that I am the least bit surprised. Votto currently owns an amazing career .310 BA and .426 OBP. His plate discipline skills are one of the best of all time. Votto excels in Outcome C – Not swinging at pitches outside of the zone.

In fact, let’s take a peek at the leaders specifically in Outcome C for 2018:

Votto finishes as the 4^th best player in the above. Towards the top of this list, we can find a number of elite batters such as Jose Ramirez, Alex Bregman & Mookie Betts.

Daniel Vogelbach leads all batters in Outcome C with a component of .479 (.713 total mPDI). Vogelbach posted a 13% walk rate last season in 102 plate appearances. To date in 2019, he has increased his BB% to 15%. Walk rate & strikeout rate (plate discipline skills) are among the first quantities to stabilize. Dan may be headed for another great season as far as plate patience goes, and he is finally getting some playing time.

Other notable players that catch my eye here are Robbie Grossman, Ryan O’Hearn and Josh Bell. Monitor the trio for their batter’s eye in 2019. Bell already has started off the season well, rebounding from a disappointing sophomore season in ’18. To date in 2019, Bell’s mPDI has surged to .782, which is elite territory.

Below are the top Outcome E batters for 2018:

Outcome E quantifies the percentage of all pitches for which the batter swung at a pitch in the zone and made contact. We see elite names at the top of this list such as Freddie Freeman, Corey Seager and Anthony Rendon.

Most notably atop the Outcome E leaderboard is utility man Jeff McNeil (.729 mPDI). Jeff posted a .329 BA in 248 plate appearances last season. To date in ’19, McNeil is hitting at a .383 clip, so he has continued his torrid contact skills. Outcome E represents the contact component of the Maddux plate discipline categories (Outcome C embodies a good eye). McNeil has a 10% strikeout rate for his career.

To close, let’s take a look at the lowest mPDI hitters from 2018:

The bottom of the mPDI boards represent the wild swingers, and the hitters who let good pitches go by.

Salvador Perez (.590 mPDI) swings at a ton of pitches outside the zone. 63% of the pitches that he sees is out of the zone, and about half of those (30% of total pitches) are swung at. In standard plate discipline terms, he has a large O-Swing%. He happens to make contact in almost two-thirds of the out-of-zone pitches, but he is swinging at far too many bad pitches. Unfortunately, Perez is injured and will be out for the rest of the 2019 season.

Jose Iglesias (.610 mPDI) lurks at the bottom of mPDI not for his wild swinging, but rather, for his non-swinging. Over 17% pitches that Iglesias sees are thrown in the zone and not swung at. He had a miniscule walk rate of 4% in 2018 in 464 plate appearances. Iglesias looks quite a bit different in ’19 – as he upped his BB% to 11% thus far, albeit with a 20% K%. It is clear that he is swinging more often than he has in the past. This may be the result of his change in teams (to the Reds), and/or working with a different hitting coach. To date, his mPDI is still below league average, but is improved at .643.

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Once again, hitting is not as simplistic as Greg Maddux implied (by the inverse of his pitching key). However, this is a great place to search for players who exhibit excellent plate discipline skills, which typically translate well from year to year. Players with a high mPDI who are due for an increase in playing time this season – are candidates to breakout or may simply post a superb on-base percentage.