Blue Jays: Has plate discipline improved this season?

TORONTO, ON - SEPTEMBER 11: Randal Grichuk #15 of the Toronto Blue Jays and Teoscar Hernandez #37 celebrate at the plate as they both score off Biggio's home run in the fifth inning of their MLB game against the Boston Red Sox at Rogers Centre on September 11, 2019 in Toronto, Canada. (Photo by Cole Burston/Getty Images)
TORONTO, ON - SEPTEMBER 11: Randal Grichuk #15 of the Toronto Blue Jays and Teoscar Hernandez #37 celebrate at the plate as they both score off Biggio's home run in the fifth inning of their MLB game against the Boston Red Sox at Rogers Centre on September 11, 2019 in Toronto, Canada. (Photo by Cole Burston/Getty Images) /

The 2019 Toronto Blue Jays ranked eleventh or worse in many of the critical measures of offence among American League teams.

One of the issues cited as an area for improvement was plate discipline. Has there been an upgrade in the early stages of the 2020 season?

Usually, I would not examine plate discipline seven games into a season because the sample size is small. However, broadcasters on Blue Jays telecasts have opined that Toronto’s hitters have demonstrated progress in this area. I was curious to see if the data confirms the reasonableness of this view.


Contributor Jim Scott introduced Jays Journal readers to the Maddux Plate Discipline Index (“mPDI”) in April 2020. Scott described the concept as follows:

"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.More from Toronto Blue Jays NewsBlue Jays: Adam Cimber, the unlikely decision KingToronto Blue Jays: Has the Shift Killed Kevin Gausman’s 2022 Cy Young Hopes?Blue Jays: What Yusei Kikuchi’s latest stumble should meanBlue Jays: Alek Manoah on pace to succeed in possible postseasonBlue Jays: Bradley Zimmer has carved himself a valuable role"

Ariel Cohen of FanGraphs developed the model in 2019. I like the mPDI stat because it captures swings on pitches in the zone (“Z-Zone”) and outside of the zone (“O-Zone”). Frequently, many people think of plate discipline solely in terms of not swinging at pitches in the O-Zone. However, I think the concept of plate discipline should capture swings at pitches in the Z-Zone as well.

Refer to Appendix I for FanGraphs’s definitions of various plate discipline metrics and the mPDI calculation.


It is essential to acknowledge that plate discipline is but one aspect of hitting. The other factors in evaluating a batter would include, but not limited to, pitch recognition, an ability to barrel a ball, plate coverage, exit velocity, launch angle, and experience. Two players, such as Randal Grichuk (0.665) and Bo Bichette (0.668), posted similar mPDIs in 2019. Yet, Bichette was the better hitter: Bichette’s wRC+ was 142, which bettered Grichuk’s 90. Plate discipline metrics (O-Zone%, mPDI, etc.) are one piece of the hitting puzzle. If Player A has better plate discipline than Player B, it does not necessarily mean that Player A is a superior hitter. It indicates that Player A is better than Player B in one aspect of hitting.

Has Toronto’s plate discipline improved since 2019?

Chart 1 shows the various plate discipline metrics for the 2020 and 2019 seasons. The components include the data for the Blue Jays, and the American League median. The 2019 New York Yankees were also incorporated into the data as a point of comparison because they scored the most runs in the American League last campaign.

The highlights are as follows:

  • The Blue Jays are swinging at a higher percentage of pitches outside the zone (32.8%) than they did in 2019 (31.8%)
  • Compared to the median, Toronto’s O-Swing% is higher in both 2020 and 2019
  • On the plus side, the home side’s contact percentage is higher on both pitches out of the zone and also in the zone
  • So far in 2020, the Jays mPDI has declined from 0.681 in 2019 to 0.676
  • The reduced mPDI means that Toronto’s batters are doing the “right thing” at a slower rate in 2020 compared to 2019
  • The Yankees had the fifth-best mPDI in the 2019 American League

While it is true that the mPDI decline from 2019 is not significant, it would be an error to claim that the Blue Jays are more disciplined at the plate thus far in 2020 than they were in 2019.

Out of interest, I looked at the average number of pitches faced per plate appearance (“PIT/PA”) this season and last (per Baseball-Reference). In 2019, the Blue Jays recorded a 3.95 PIT/PA; the median was also 3.95. In 2020, Toronto has averaged 3.84; the median is 3.97. However, the reduction in PIT/PA from 2019’s mark maybe because the Jays have seen an increase of in-zone pitches in 2020 (43.2%) from 2019 (40.9%). The fall in PIT/PA could be attributable to more strikes to swing at rather than a deterioration of plate discipline.

How have the returning Blue Jays from 2019 performed?

Table 1 is a presentation of the 2020 and 2019 mPDIs of Blue Jays, who were on last season’s roster. The highpoints are as follows:

  • Bichette’s mPDI has declined from 0.668 to 0.627
  • The mPDI reduction is because Bichette is swinging at more pitches out of the zone in 2020 (46.4%) compared to 2019 (38.6%)
  •  Of the listed players, Lourdes Gurriel Jr. has seen the second-largest mPDI drop; 0.647 in 2019 to 0.566 in 2020
  • The causes of this plate discipline deterioration are twofold
  • First, Gurriel Jr.’s O-Swing% was 38.6% last season and has risen to 48.6% so far in 2020
  • Second, he is swinging at pitches in the strike zone at a lower rate this campaign (61.4%) compared to 2019 (69.6%)
  • Teoscar Hernandez, whose mPDI was above the median in 2019 (0.710), has been less disciplined in 2020 at the plate; his mPDI has sunk to 0.611
  • This decline is attributed partly to his swinging at more pitches out of the zone (30.3% in 2019; 37.9% in 2020)
  • The other factor is the lower rate of swinging at pitches in the strike zone (60.0% in 2020 and 72.9% in 2019)
  • On the positive side of the ledger, Rowdy Tellez has improved his mPDI from 0.622 to 0.663
  • Although Tellez’s mPDI remains below-median, he accomplished his mPDI gain by swinging at more in-zone pitches in 2020 (75.7%) than he did in 2019 (64.9%)
  • Vlad Guerrero Jr. also has demonstrated better overall plate discipline in 2020, as evidenced by a higher mPDI in 2020 (0.721); it was 0.697 last season
  • While his O-Swing% has increased from 31.6% to 34.3% in 2020, Vladdy Jr. is swinging at more strikes (81.8%) compared to 2019 (71.9%)

Overall assessment

Better plate discipline in 2020 compared to 2019 was a target set by many Blue Jays fans, and probably the members of Toronto’s baseball organization. An improved approach at the plate should translate into a higher BB% and batted ball profile, not to mention a reduced K%. So far, in 2020, BB% has sunk to 7.0% (12th in the AL) from 2019’s mark of 8.4% (10th). OBP is languishing at the 0.285 level (13th) this season; in 2019, it ranked 14th at 0.305. The 2020 sample size is small, which precludes any reasonable plate discipline projections at this point. Nevertheless, it is rational to conclude that any claim that the 2020 Blue Jays are collectively more disciplined at the plate compared to 2019 runs contrary to the relevant data.

dark. Next. Blue Jays: The Good, the bad and the ugly

The last word

The 2020 Blue Jays have not shown plate discipline development thus far this season. As a unit, there has been a slight worsening of the underlying metrics. While Toronto is a young group that can make advancements in its approach, it is too soon to credit the team for improved plate discipline at this point in the season.