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Why Not Every Decision to Swing is Created Equal


In this installment of a series on advanced baseball metrics, we examine what discipline+ tells us about the versatility of Mike Trout, Juan Soto and Ronald Acuna Jr. 

By: Analyst Staff

From the perspective of a hitter digging into the box, every pitch initiates a critical split-second thought process that could impact the entire at-bat: to swing or not to swing.

Some websites dedicated to analytics traffic in metrics like a batter’s swing rate, often on pitches both in and out of the strike zone, in an effort to measure discipline. That’s not good enough. Not all decisions on a given pitch are created equal.

For example, a batter protecting the plate with two strikes who swings at a pitch just out of the zone shouldn’t be penalized the same as a hitter whiffing on a pitch way off the plate on a 3-1 count.

Juan Soto ranked third in the majors in discipline+ in the shortened 2020 season. (AP Photo/Julio Cortez)

As previously mentioned, not every decision to swing is created equal.

Discipline+ is formulated by first assigning a value to every pitch based on velocity, movement, location, and count. From this, the hitter is given a value of his own based on his decision to swing or take the said pitch. A batter gets more credit for swinging at a pitch that has a 90.0% chance of being called a strike than if he decides to cut loose on a pitch that has a lower probability.

So considering that the league average is 100, here’s who finished in the top 10 last season:


(minimum 180 plate appearances)
1Brandon NimmoMets139
2Max MuncyDodgers134
3Juan SotoNationals131
4Carlos SantanaIndians131
5Marcus SemienA's129
6Mike TroutAngels129
7Christian YelichBrewers126
8Yoshi TsutsugoRays124
9Ronald Acuna Jr.Braves124
10Anthony RendonAngels124

As expected, some of the game’s most patient hitters are in the top 10. But there are plenty of sluggers there too like Juan Soto, Ronald Acuna Jr. and Mike Trout. Soto had the fewest plate appearances per walk (4.8) in the majors and hit .351 with 13 home runs in 154 at-bats.

Carlos Santana and Christian Yelich had down years, but plate discipline wasn’t to blame as they finished second (47) and third (46) in the majors in bases on balls.


Data analysis provided by Lucas Haupt.