My writings about baseball, with a strong statistical & machine learning slant.

Sunday, May 2, 2010

Pitchers: Eight Different Kinds

I often read that: "pitcher X is similar to pitcher Y, so he should a) have similar performance b) age at the same rate c) experience a similar injury history." Sometimes the comparisons are based on height, weight, handedness, or otherwise, but usually we think of pitchers as being similar, if they deliver similar performance. Without advanced stats, we might consider pitchers similar if they had similar won-loss records or had similar strikeout rates. Now, some of us might think of pitchers as being similar if they are fly ball pitchers or ground ball pitchers, or if their fastball moves inside or outside to right-handed hitters. However, these categories all seem to reflect performance, to varying degrees.

My goal is to classify pitchers, in a performance-neutral way, based on the pitches that they throw. I will present eight categories that comprehensively classify pitchers into types, based almost exclusively on the percentages that they throw their pitches (collected for 2002-2009 by BIS and for 2007-20010 by PitchF/X, all viewable on FanGraphs).

Why Classify by Pitches?

This is not to say that classification by factors other than pitch selection isn't interesting. We already classify pitchers by role (starter/closer/setup man) and by performance, in casual baseball conversations. However, I think it would be useful to classify pitchers by how they pitch, rather than by their results. In order to ask questions like "how do curveball pitchers age relative to other pitchers?" and "do fastball/slider pitchers make good starters?" it is necessary to have a way to classify pitchers based on what they throw, in a comprehensive and non-arbitrary way. (Declaring that a pitcher is a curveball pitcher if and only if he throws 25% curveballs seems too arbitrary, and unnecessarily restrictive.)

Why Eight Categories?

Baseball Info Solutions (BIS) tracked & classified a large number of pitches for 2002-2009, giving us insight into how each pitcher's repertoire broke down over those years. This is the data that I use to derive my classifications. Since 2007, MLB has contracted Pitch F/X (same guys who do the virtual first-down line for NFL games) to track pitches in all major league stadiums. Their data is more comprehensive (Pitch F/X also tracks two-seam (sinking) fastballs), but it is available on a much smaller sample. (All of this data is hosted on FanGraphs and can be viewed for free.)

BIS recognizes seven pitches: fastball, slider, cutter, curve, change-up, splitter and knuckleball. A few months ago, I developed a system to determine which of these were a pitcher's "core pitches," which were "secondary pitches," and which were pitches that he did not really throw (even if they had pitch counts over 0% according to BIS. Consider a pitcher who throws 70% fastballs, 25% sliders, and 3% change-ups and 2% curveballs. His core pitches are fastball and slider, while the others are not relevant. Now if he threw 70% fastballs, 20% sliders, 9% change-ups and 1% curveballs, we might says that fastball and slider are still his "core pitches," but "change-up" is now a "secondary offering." I classify pitchers based on these distinctions.

Over the course of a season, almost all pitchers' repertoires can be described as:
  • core offerings: Fastball + 0-2 other pitches
  • secondary offerings: 0-3 other pitches 
Actually, most pitcher seasons fall into an even tighter rule:
  • core offerings: Fastball + 1 other pitch
  • secondary offerings: 0-2 other pitches
Therefore, the simplest way to describe a pitcher's repertoire is to name his core offering (ie the pitch he throws most often, not including his fastball). There is a strong correlation between the core offering and his likely secondary pitches. Pitchers who throw lots of curveballs tend to throw some change-ups, as well. Pitchers who throw splitters usually also throw sliders.

Since there are six possible pitches available as core offerings (not including the fastball, which everyone throws), there are six obvious categories. However, the knuckleball is so rare in today's game that I do not create a category based on it. Instead, there are five obvious categories (slider, cutter, curve, change-up, and splitter). In practice, more categories are needed, since not all pitchers who throw fastballs and sliders are truely similar, etc.

Using a simple clustering algorithm (SimpleKMeans using manhattan distance for the "core" and "secondary" pitches), I came up with seven categories. This was enough to get a category focused on cutters. I still needed a category for splitters, so I created one manually (by computing a centroid for all pitchers who throw a splitter). Then I tuned the weights and added other small considerations, so that the selection criteria for the categories were more intuitive.

Eight Pitchers

Here are the eight pitcher categories that I came up with, along with explanations, examples, and obvious tendencies.

Type 0: Changeup (Slider & Curve backup)
(14% of pitcher seasons)

The types start with 0, since nerds number their days 0-6.

A typical type 0 pitcher throws 61% fastballs, 18% change-ups, 11% sliders, and 6% curves. They are most similar to type 5 pitchers, although they are more likely to throw the slider, less likely to throw the curve, and less likely to be starting pitchers.

Sucessful type 0 pitchers include:
  • Pedro Martinez (until 2006, when he became a type 5)
  • Felix Hernandez
  • Johan Santana (until 2007, when he became a type 5)
Type 1: Cutter (no common backup)
(4% of pitcher seasons)

A typical type 1 pitcher throws 56% fastballs, 20% cutters, and various other pitches depending on the pitcher. Type 1 pitchers are not similar to any other pitchers, but they are more likely to throw curveballs that sliders, which makes them somewhat similar to type 4 and type 5 pitchers. Type 1 pitchers are likely to be starters, but some very successful closers are type 1 pitchers.

Successful type 1 pitchers include:
  • Mariano Rivera
  • Jon Lester
  • Andrew Bailey
  • Roy Halladay since 2007 (previously a type 4 pitcher)
Type 2: Slider (Change-up and Curve backup)
(18% of pitcher seasons)

Type 2 pitchers are fastball/slider pitchers, who also throw other pitches at non-negligible rates. A typical type 2 throws 62% fastballs, 18% sliders, 10% change-ups and 7% curves. Type 2 pitchers are similar to type 6 pitchers, but type 2 pitchers are those that also have secondary offerings. Type 2 pitchers are more likely to be starters than type 6 pitchers, and also don't throw as hard.

Successful type 2 pitchers include:
  • CC Sabathia
  • Zack Greinke
  • Jake Peavy
Type 3: Curve & Slider (no common backups)
(4% of pitcher seasons)

Type 3 pitchers throw both a curve and a slider frequently enough for them to both be considered "core pitches." A typical type 3 pitcher throws 57% fastballs, 18% curves and 16% sliders.

When I first saw this category, I thought these might be pitchers who throw a "slurve," or those pitchers who switched breaking pitches within a season. Those would end up in this category, to be sure, but successful type 3 pitchers recently included some very good pitchers:
  • Chris Carpenter (2005 and 2009)
  • Adam Wainwright (2005 and 2009, type 2 in between)
  • Matt Morris
Type 4: Curve (with Change-up backup)
(16% of pitcher seasons)

A Type 4 pitcher throws 64% fastballs, 21% curves, and 9% change-ups. These pitchers typically throw very few sliders. They are most similar to type 5 pitchers (who throw more change-ups), although a type 4 pitcher can become a type 3 pitcher if he starts to throw more sliders. 

Successful type 4 pitchers include:
  • Roy Halladay before 2006 (now he's a type 1 pitcher)
  • Roy Oswalt
  • Ben Sheets
  • Justin Verlander 

Type 5: Change-up (Curve backup)
(8% of pitcher seasons)

A typical type 5 pitcher throws 62% fastballs, 23% change-ups, and 12% curves. Type 5 pitchers are similar to type 0 pitchers in that both rely heavily on the change-up. However type 5 pitchers throw more curveballs, while type 0 pitchers throw silders. Also type 5 pitchers are the softest-tossing category that I considered, with a typical average fastball of only 89.1 mph.

Successful type 5 pitchers include:
  • Cliff Lee (except 2007)
  • Tim Lincecum
  • Trevor Hoffman from 2002-2007 (currently type 0)
  • Tom Glavine
  • Greg Maddux
Type 6: Slider (no backup)
(32% of pitcher seasons)

Type 6 pitchers are your prototypical fastball-slider flame throwers. A typical type 6 pitcher throws 66% fastballs and 24% sliders, with 5% change-ups and no curveballs. Type 6 pitchers are similar to type 2 pitchers, but they throw fewer secondary pitches. When a slider-throwing type 6 pitcher starts throwing a secondary pitch, be becomes a type 2 and vice versa.

Type 6 pitchers are the most common pitcher in MLB today, representing 32% of pitcher seasons from 2002-2009. They are the most common relief pitchers in baseball, but some successful starting pitchers were also type 6 pitchers. Type 6 pitchers often become type 2 pitchers, but here are some successful type 6 pitchers who have remained so throughout their careers:
  • Randy Johnson
  • BJ Ryan
  • Joe Nathan
  • Brad Lidge
  • Josh Johnson 
Type 7: Splitter (Slider backup)
(4% of pitcher seasons)

Type 7 pitchers have become a rare breed, still I felt compelled to force a category for them. Splitter throwers have been some of the most successful pitchers of the last 20 years, so even if there are very few left in 2010, they are still worth separating them from the type 6 pitchers that they most resemble. A typical type 7 pitcher throws 60% fastballs, 15% splitters, and 10% sliders. 

Successful type 7 pitchers include:
(4% of pitcher seasons)
  • Roger Clemens
  • Curt Schilling
  • Hideo Nomo
  • Armando Benitez
Of course, Bruce Sutter is most famous for "inventing" the splitter. His version was thrown very hard, as a fastball with movement. Pitchers before him threw a split-finger pitch called a forkball, but that was more of a change-up. For some reason, the splitter has lost its popularity in the last few years, despite some of the best pitchers in recent history being avid proponents.

Typical type 7 pitchers throw even harder (91.5 mph average fastball) than typical type 6 slider-throwers (91.1 mph average fastball). This suggests that only hard-throwing slider-type pitcher ever become splitter-throwers. For some reason, recently very few type 6 pitchers have attempted this transition. There are probably good baseball reasons why this is has been the case, but I don't know what those might be. I'm an expert on machine-learning classification, not pitcher mechanics!

Hopefully the classifications and examples sound intuitive & intriguing. FWIW, here is a list of all 2009 pitcher seasons (over 20IP) with type classification (seasons ranked by pitcher VORP).

Despite the fact that these classifications are made without any consideration for performance, there are definitely differences in average performance between the eight categories of pitchers. I will follow up to discuss some of those differences. However, I think the most useful aspect of my categories is their descriptive quality. Although they rely (almost exclusively) on comparing % breakdowns of pitches thrown, the categoires can be described simply and intuitively. Not all pitchers fit neatly into a category, but very few pitchers don't belong in any category at all. I have made simplifications by ignoring knuckleballs and different fastball types, but without these simplifications, the list of categories might be even longer. My aim is to be insightful, but simple. 

I'm working on a site to host some of my data, including profiles for all pitchers, and their classifications by pitcher type. However, this will take some time. If you are interested in the type breakdown for a specific pitcher, take a look at this list, or contact me! 


  1. Type 3 pitchers seem to be Cardinals even though they're just 4% of pitcher seasons they made up 12.5% of the Cardinals' 16-man staff last season. Also, while Type 2 pitchers make up just 18% of the MLB population, they made up 31.25% of St. Louis' staff last season. They seem to be doing that by avoiding Type 0, 4 and 7 pitchers. I wonder if this could be a personal preference by pitching coach Dave Duncan. Do you have data that suggests some MLB teams look for certain types of pitchers and/or convince pitchers to use a certain percentage of their stuff?

  2. That's an interesting idea. Sample sizes are too small, but worth taking a closer look for something suggestive. I can publish the list of pitchers by categories and teams. Should have a short post about this later today.

    Thanks for the comment!

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