Blue Jays and the Maddux Plate Discipline Index

The Maddux Plate Discipline Index (mPDI) developed by Ariel Cohen is a simple metric to measure a player’s plate discipline.  What does it say about the Jays hitters?

Greg Maddux, the Hall of Fame pitcher with the Braves and Cubs, once said that 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.  A pitcher who can do that, he proposed, will do well.

In a 2019 Fangraphs article, Ariel Cohen suggested that this concept could be also applied from the hitter’s perspective.  A hitter who swings at pitches inside the strike zone but does not swing at pitches outside the zone would do much better than a hitter who does the opposite.  Cohen used this theory to develop a metric, the Maddux Plate Discipline Index (mPDI), which is basically the percentage of the time that a player did the “right” thing – either swung at a pitch in the zone or did not swing at a pitch outside the zone.

This metric is a simplification, as most metrics are.  There are times, early in the count with strikes to spare, when it is to a hitter’s advantage to not swing at a strike, just as it is sometimes good for a hitter to swing at a marginal ball that is right in his “sweet spot”.    As such, a no-hitter gets a 100% score – the very best hitters can flirt with .800 (80%), while the league average is around .680.

Toronto Blue Jays

So, caveats done, what can this metric tell us about the 2019 Jays?  Have a look at this table.  For reference, an O-Swing% is the percentage of pitches outside the strike zone that a player swings at, with an MLB average of 31.6%.  A Z-Swing% is the percentage of pitches inside the strike zone the player swings at (average 68.5%), and Zone% is the percentage of pitches that the players see that are in the strike zone (average 41.8%).

The mPDI star of the 2019 Jays is clearly Cavan Biggio.  His .754 (75.4%) not only led the Jays but was third in baseball (of the 360 players with minimum 200 PAs).  His prominence is largely due to his holy-cow-Batman  15.8% O-Swing% – second only to Logan Forsythe‘s 15.3%.  Bo might have ranked even higher were it not for his below-average 63% Z-Swing%.  If Cavan could keep up his keen eye on balls but be a bit more aggressive on strikes, he could be even more of a plate discipline beast than he is now.

It is also heartening that Danny Jansen, Teoscar Hernandez and, Vladimir Guerrero Jr. all have above-average mPDIs.  This bodes very well – often, younger players (especially rookies) have difficulty holding back.  That our young guns were able to do so is a very good sign.

Bo Bichette and Randall Grichuk both had mPDIs in the 67% range, slightly below league average.  In both cases, this resulted from a higher than average O-Swing%.  It is possible that both players were pressing in 2019 – Bo as a rookie, and Randall in an attempt to duplicate his excellent H2 (second half) of 2018.  With luck, these O-Swing% should both decline in 2020.

Lourdes’ 64.7% was troubling, particularly given that his O-Swing % was even higher in the second half.  Could it be that his struggles at the plate (he had a 152 wRC+ in the first half but only 78 in the second) caused him to over-compensate?  This will definitely be a factor to watch in 2020.

And of course, Rowdy Tellez‘ 62.3% was pretty bad – not only last on the Jays but 349th of 360 players with 200+ PAs in 2019.  It is no secret that Rowdy has issues with plate discipline.  His 39.3% O-Swing% and his Z-Swing% of 64.9% are both worse than average.  So he is swinging too often on balls, and not often enough on strikes.  Could this be a vision problem?  Or a perception problem?  Or did Rowdy feel that he was under too much pressure to perform in 2019 – a Dalton Pompey redux?  Whatever the reason, this is an issue that Rowdy (and the Jays) will need to address.

The bottom line

You might expect a young team like Toronto to do poorly in the mPDI metric, as younger players are often not the most patient at the plate.  Relative to that expectation, the Jays generally did very well in 2019, and with increased experience and maturity there is cause for optimism for the future.