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Visualizing the Pirates' plate discipline

Rick Osentoski-USA TODAY Sports

This is an article about flailing. Flail-iness.  Flailitude. PedroSandovalism. This. Whatever you want to call it.

I wrote it because I've seen many comments similar to this one:

"This flailing against the Reds' garbage pitchers needs to stop."

. . . and I thought it would be interesting to have a conversation about how we measure to what extent hitters do, in fact, flail. Or, more broadly, how we measure hitters' approaches at the plate.

The amount of publicly available baseball data has grown so rapidly that our collective ability to distill it into meaningful information lags behind. It's only been over the last ten years or so that metrics like on-base percentage have entered the public lexicon, and most non-FanGraphs readers aren't tracking things like walk and strikeout rates. The best available statistics for capturing hitters' flail-iness (or lack thereof) are probably Z-Swing % and O-Swing %, the percentages of pitches inside and outside the strike zone (respectively) at which a hitter swung.

Yet neither of these metrics has acquired the power of language. In other words, when I say Player X is a .300 hitter, or had 25 home runs last year, or stole 30 bases, or even (to a lesser extent) had an .800 OPS, that means something to most baseball fans. People know, roughly, what a good batting average is, how many home runs you need to be considered a power hitter, etc . . .

But if I were to tell you that Player X has a 78% Z-Swing % and 42% O-Swing % it probably wouldn't mean much to you. Putting these numbers in context can help us understand hitters' approaches at the plate.

The graph below displays plate discipline data for every hitter with at least 200 plate appearances as of August 4th. It plots their aggressiveness on pitches inside the strike zone (Z-Swing %) against their aggressiveness on pitches outside the strike zone (O-Swing %). The grey lines and shading indicate the median and 25th/75th percentiles for each metric.

A couple things are clear--Adam Jones swings at virtually everything, while Ben Zobrist swings at virtually nothing. Andrew McCutchen is aggressive within the zone, yet doesn't swing at bad pitches. Omar Infante is in trouble. The median MLB semi-regular hitter swings at 68% of pitches within the zone and 32% of pitches outside of the zone.

I grouped and color-coded the hitters to make the chart a bit easier to follow. The names are, I admit, not the best--if it helps, think of them in terms of the hitters who best exemplify the different approaches--maybe the Aggressive group is the Adam Jones group.

It's (hopefully) interesting to look at the numbers this way, but while it helps us understand hitters' plate approaches it doesn't answer deeper questions about the interplay between plate discipline and success as a hitter. We could probably, in the absence of any other information, make some pretty good guesses about how the different groups tend to perform. We might think that the 'Aggressive' and 'Non-selective' groups don't draw a ton of walks, while the 'Selective' group tend to be the best hitters. In general, we'd expect hitters who swing at more bad pitches to perform worse.

Here's what we actually observe:

A couple thoughts:

  • Yup, all four groups have a mean wRC+ above 100. This is possible because we're only looking at hitters with 200+ PAs, who tend to be better than the population as a whole.
  • The Selective group is by far the most productive but, surprisingly, the Non-selective group does just fine--the gap between the other three groups is much smaller than you'd think.
  • Passive hitters tend to see more balls in the zone.
  • The Selective group both hits the ball with the most authority and strikes out the most.
  • The relationship between O-Swing % and overall offensive production isn't as clear as you'd expect.
On that last point, while there is a correlation between O-Swing % and offensive production, it isn't particularly strong. There are plenty of bad hitters who show restraint, and several productive hackers. See below:

So where do the Pirates' hitters fall in all of this?

If we take the first graph, the one that shows the entire continuum of hitter aggressiveness and plate discipline, and call out only the Pirates players, this is what we get:

Probably nothing all that surprising, if you've been watching the team all year. Starling Marte is among the most aggressive hitters in the entire league; Jung-Ho Kang and Francisco Cervelli fall at the opposite end of the spectrum. Andrew McCutchen is extremely patient outside the strike zone and aggressive within it--and ideal combination. There's not much of a pattern here--Pirates hitter fall all over the spectrum. And indeed, if we look at the same metrics on a team level, the Pirates are collectively pretty average:

Some teams seem to have consistent organizational philosophies with regard to hitting. The Athletics are patient. The Orioles swing at everything. The Giants seem to do everything well. The Pirates don't appear to have a consistent 'type'--they're willing to accommodate both the Martes and Kangs of the world, recognizing that there are multiple paths to success.

This isn't a rigorous, exhaustive treatment of this topic, and I don't think it lends itself to any bold analytical conclusions. Certainly, it's easy to take issue with using swing percentages in this way. For instance, you could argue that we're actually examining two discrete skills here--pitch recognition and decision-making, and that it's probably impossible to accurately separate them. We're also treating all balls inside the strike zone as the same, and all balls outside the strike zone the same, even when this is clearly not the case.

But I think it's interesting just to see how the data confirms (in this case) our perceptions--Josh Harrison actually does swing at a ton of terrible pitches, and McCutchen actually has excellent plate discipline. It's a small sample size, but Jaff Decker would grade out as the most passive (or patient, depending on your perspective) hitter in the entire league. These all seem like useful pieces of information that help us understand the game in a unique way. With the continuing provision of more and more data to the public, I'm sure it won't be the last time we all learn something new.