The Beginner’s Guide To Plate Discipline

At its heart, baseball is a battle to control the strike zone. There are plenty of other things going on, but the origin of the action is over the plate. Good hitters make good decisions about when to swing and when to take and good pitchers attempt to negatively impact that decision-making process. As the importance of walks and working counts became clear over the last generation, hitters who knew the zone and pitchers who could generate swinging strikes became very popular.

Throughout history, batters have been judged by their results. Things like batting average and RBI have given way to wOBA and WAR, but in general the average fan cares about the outcomes rather than the process. Plate discipline numbers are inherently process based. You don’t get credit in the box score for taking a pitch just off the plate, but taking a pitch just off the plate is probably going to help you do things that lead to runs, like walking and getting good pitches to hit.

So the inherent question we’re always asking is “does X help the team win?” When we talk about something like FIP, we’re really talking about pitchers who do things that tend to lead to run prevention. When we talk about UZR, we’re talking about fielders who keep runs off the board. And preventing runs on the mound or in the field makes it more likely that your team will win.

Offensively, the goal is to get on base and to drive the ball with authority in order to hit for extra bases. Each particular situation comes with a lot of nuance, but the basic goal of any batter is to reach base. So the question we want to ask specifically is “what leads to getting on base?”

Getting on base requires that you walk or that you get a hit, and walking generally requires that you have a good eye and getting a hit generally requires that you make solid contact. Of course you can do both while doing neither, but on average, you either need to work the count and take pitches outside the zone or you need to clobber the pitches that are close enough to hit. Doing either requires that you have a good command of the zone.

That’s where plate discipline comes in. Imagine a batter with immense raw power. If you throw him batting practice fastballs, he can deposit them into the seats with relative ease. He has no problem bringing the barrel to the ball and when he connects the ball goes on a nice little trip. In fact, he can handle tough pitches too. You tell him 95 mph is coming on the corner and he can crush it. You throw an Adam Wainwright curveball and he can drive it. He’s got an 80 hit tool and an 80 power tool, let’s say.

But there’s a third dimension at the plate that we also want to consider. This hypothetical player has the ability to hit and hit well, but he might swing at everything whether it’s in the zone or up around his eyes. With his raw ability, he can probably get away with that, but he could elevate his game even further if he laid off pitches which were harder to hit and swung at pitches that were easier to hit. You get a finite number of plate appearances and you want to get the most out of each one.

In other words, plate discipline is about swinging at pitches that you can handle and taking pitches that you can’t. If you swing at pitches way off the plate or take pitches in your wheelhouse, you’re not maximizing your offensive potential, and you generally want to do that. So while there’s no value directly tied to swinging at a low number of pitches outside the zone, it’s typically a good quality because it’s pretty hard to hit those pitches with a lot of oomph.

There are a variety of plate discipline statistics that measure different parts of the overall picture, and I should note that we have two sets of plate discipline stats that use Baseball Info Solutions data and PITCHf/x data. The concepts are the same, the inputs just vary based on who is coding the data.

The stats are divided up in two dimensions. There are swing rates and contact rates and in-zone numbers and out-of-zone numbers. There’s no perfect swing rate, either in or out of the zone. Each individual pitch and situation is different. The same is true for contact rate. Even though swinging and missing is never a good outcome overall, there are times when you’re prefer to swing and miss because if you made contact it would be weak contact and your PA would be over.

In some ways, using the strike zone as a defining area can be a little misleading because each batter handles different parts of the strike zone in different ways. Mike Trout dominates the bottom of the strike zone, so he can swing at pitches below the zone without hurting his production, whereas hitters who have a swing better suited for the top of the zone should definitely think about laying off those low pitches.

It may be frustrating to learn that you can’t simply look at the numbers and know exactly what’s happening, but the numbers are useful nonetheless. Swing rates and contact rates tell you about the hitters overall approach. Are they aggressive? Are they good at getting the back to the ball?

We’re a year or two away from having a good handle on understanding where hitters make “good contact,” but even then everything is very dependent on the situation and the pitcher. The break even point on swing/don’t swing changes as the game changes. The overall takeaway is not that there is an ideal set of plate discipline stats, but that deciding which pitches to swing at is a very important compliment to one’s ability to physically hit a baseball.

When you hear someone talk about a hitter’s discipline at the plate, it’s not really because patience is inherently good and aggression is inherently bad. Discipline doesn’t mean passivity, it means swinging at the right pitches and the ability to make those split section decisions is a really important skill that you want to consider when evaluating a player.