Aces Don’t Exist: Flexible Elites

In 2018, MLB teams employed 799 distinct pitchers to fill 892 spots, which is quite a few hurlers. That’s an increase for 2017, when 754 distinct pitchers filled 839 roles. The 30,000 foot overview of this progression suggests that each MLB team had room for at least one additional pitcher in 2018 (it could have been a player like Erik Kratz or Mike Zagurski or Corbin Burnes, depending on how you’d like to look at this equation). Not every pitcher included in these counts worked both seasons, so in total, MLB teams required 998 distinct pitchers to complete their 2017 and 2018 workload. These workload requirements produce a diversity of roles, and this feature will hopefully demonstrate that there are multiple categories of Runs Prevention success, such that a club like the Brewers can indeed excel without “a true ace.”

In 2017, the 84th best pitcher averaged approximately 10 runs prevented, establishing the 90th percentile performance point; in 2018, the 89th best pitcher averaged approximately 10 runs prevented, suggesting that the 90th percentile performance did not change. If you like false certainty, even here the decimal points look similar, as the 2018 cut-off was 10.221 average runs prevented versus 10.229 average runs prevented in 2017. Let’s call it 10 Average Runs Prevented for fun…

 Exploring Runs Prevented || Aces Do Not Exist || Rotation Spots || Third-Time Charmers

What is Runs Prevented? Runs Prevented is a relatively basic statistic that attempts to measure the quality of a pitcher’s performance within their run environment (which is roughly the combination of their ballpark, which can impact scoring, and their league, which can impact the mix of talent faced). Measuring pitching is difficult because unlike batting stats, you cannot simply “count up” with pitchers; a team that scores 750 runs is  likely better than a team that scores 700 runs, but a team that allows 650 runs is likely better than a team that allows 700 runs. So, assessing pitching in a “run environment” requires accounting for that “negative space” (in this example, between 650 RA and 700 RA). Throughout the 2018 season, I developed an Average Runs Prevented statistic that attempted to track variance in park factors (between Baseball Prospectus Pitching Park Factor and Baseball Reference Park Factors) as well as variance in league environments. Usually these differences did not amount to much, but tracking them is an important step to understanding variance and reflecting uncertainty in the Runs Prevented metric: I can say “Jeremy Jeffress prevented 24.63337 runs in 2018,” or I can say “on average Jeremy Jeffress prevented between 23 and 26 runs in 2018,” or I can say, “Jeffress prevented 25 runs.”


…The short lesson: if you’re an MLB pitcher that prevented 10 runs, you’re really, really good. This holds regardless of role. In 2018, the top ten percent of all MLB pitchers featured 53 starting pitchers and 36 relief pitchers, which is quite a swing in favor of starting pitching quality; in 2017, these 90th percentile pitchers featured 41 relievers and 43 starters. What’s important here is to takeaway that great relievers prevent enough runs to break into the top threshold of league performance; Runs Prevention need not categorically be dominated by starting pitching, which is one reason the Brewers did not need a starting pitcher at the trade deadline (thanks to their strong bullpen).

Aces are more difficult to define than “Average Runs Prevented.” I can at least give you a math equation for Average Runs Prevented at the end of the day. But an “ace” can be many things;


  • Some fans want an ace to be a breathy “dude” or “guy”, a “stud” atop the rotation. File this one under the debate about playoff dudes,  “Do you want Jhoulys Chacin and Wade Miley to pitch in the playoffs?” (Yes, actually);
  • Some people ascribe to a theory than an “ace” is simply the best pitcher on each MLB team (e.g., 30 teams means 30 aces, and no team can have more than one ace);
  • Scouts like to define “aces” in very rigorous ways, focusing on the quality of a pitcher’s stuff (typically better than average fastball and off-speed or breaking ball is the minimum stuff requirement for acehood), as well as their command (must be great), and probably their frame, too (it’s good to project innings from an ace). But I take it that this is not how most fans mean “ace,” and it’s also no fun because aces rarely exist under this mold (#EveryoneIsAMidRotationStarter);
  • Finally, there’s the simple stipulation that an “ace” is an elite starting pitcher, at the top of the league in terms of performance (presumably, hopefully, for multiple consecutive years. For example, this is why people say Clayton Kershaw is an ace, but Junior Guerra is not. I used to be sure of the importance of consistency, but….well, aces hardly exist under this requirement, either).


In this analysis, I am going to demonstrate two of the shortcomings with the idea of using performance to define the concept of an ace.

(1) Threshold of Greatness. Seeking the 90th percentile of MLB pitchers seems like a very high standard; it’s quite literally the top of the league, but includes a large enough group of pitchers to make meaningful comparisons. What I mean by this is, if we used a more strict threshold, there really would not be any aces; take the Top 10 pitchers by Runs Prevented in 2017 and 2018, for instance:

2017 2018
Corey Kluber Blake Snell
Max Scherzer Jacob deGrom
Chris Sale Kyle Freeland
Clayton Kershaw Aaron Nola
Stephen Strasburg Chris Sale
Gio Gonzalez Max Scherzer
Carlos Carrasco Justin Verlander
Luis Severino Trevor Bauer
Robbie Ray Corey Kluber
Zack Greinke Mike Clevinger

I rather like this, in the sense that it demonstrates that aces do not exist, but it doesn’t pass the eye test. If you’ve designed a threshold where only Corey Kluber, Chris Sale, and Max Scherzer are aces in 2017 and 2018, but not Kyle Freeland, Justin Verlander, Clayton Kershaw, Blake Snell, or Jacob deGrom, you’ve probably missed the threshold and poorly defined the concept.

An interesting problem arises with using the 90th percentile threshold, however: starting pitchers no longer dominate the proceedings, and 10 Average Runs Prevented is the measure for an ace. This surely won’t do for Brewers fans, for example, as 2016 Junior Guerra is absolutely, positively an ace under this regard; interestingly enough, so are 2018 Jeremy Jeffress, Josh Hader, and Wade Miley. Of course, this is conceptually interesting because Brewers fans absolutely did not want Miley to work in the rotation, and he was an “ace,” it turns out. Really, a truly fantastic pitcher, better than 90 percent of the MLB. Jeffress and Hader are difficult in terms of “acehood” because they do not start ballgames; but it’s not clear to me than an ace must start ballgames.

Consider the false controversy to open 2018, regarding whether or not Josh Hader should start or work in his relief role; as a starter, the concern is clearly that Hader does not have the command or pitching profile to work through a batting order multiple times, and based on news throughout the season, his delivery is probably too high effort to withstand a starting workload. But that was never the debate; the debate among angry fans was, “Josh Hader should start because the Brewers must see whether he can be an ace.” Ironically, based on the 90th percentile threshold of Runs Prevented, Hader is an ace, undoubtedly so (in fact, he’s better than 95 percent of the league in 2018; Jeffress was even better, beating 98 percent of the MLB). In this debate, it seems that the ideal of “finding an ace by making sure they can start games” misses the concept of what an ace ought to do (“consistently prevent runs at an elite level”), and so the Hader starting pitching controversy falls by the wayside.

Both Hader and Jeffress are aces, on this model. As they should be.


(2) Consistency. Josh Hader showed flashes of brilliance during the 2017 season, in which he established a clearly valuable MLB “floor” performance level, and raised debates about how high the “ceiling” performance could be. I gather this is in part why fans wanted so badly to see Hader start; when he began his career in the bullpen, it was certainly due to his tough season at Triple-A Colorado Springs, where he lost his breaking ball and had his stuff and command profile back-up a little bit. Yet Hader excelled in his MLB debut role, working through some command issues by offsetting bad outings with a dozen scoreless multi-inning appearances. Hader prevented between 13 and 14 runs in 47 and 2/3 innings.

2017 & 2018 Pitchers Number
Number of Pitchers Working Both Seasons 639
Average (Absolute Value) Runs Prevented Change (2017 to 2018) 9
Average (Absolute Value) Innings Pitched Change (2017 to 2018) 33

Any team will receive considerable value from a pitcher that prevents more than 10 runs in a season; no team would balk at a chance at 90th percentile runs prevention production. Yet, there is more value to be had from pitchers that can repeat the feat in consecutive seasons. For example, 639 pitchers worked in both 2017 and 2018 seasons, with considerable variance in their production. The typical back-to-back pitcher in 2017 and 2018 found their runs prevented total shift by at least nine runs, with their innings pitched total fluctuating by 33. If you ascribe to the rule of thumb that approximately ten runs are equivalent to “one win” when balancing Runs Scored and Runs Allowed, the MLB pitchers that worked in 2017 and 2018 fluctuated enough to produce as many as 575 total wins (or losses), depending on how teams balanced their resources (as a side note, this is one reason organizations should not ever tank, and should always try to compete: there are always tons of runs that can be “captured” every year through trades, free agency, and player development fluctuations).

This is where I believe fans and analysts turn against “one year aces” like Junior Guerra. The idea is not that Guerra was not valuable to the Brewers in 2016, but that it is more valuable to have a pitcher that a team can “depend” on to produce 90th percentile performance year-in, and year-out. The trouble is, these pitchers do not readily exist. Looking at 2017 and 2018, here are the pitchers that were able to produce 10 Average Runs Prevented (or better) in both seasons:

Consistent Pitchers 2017 Average Runs Prevented 2018 Average Runs Prevented
Jacob deGrom 13 49
Aaron Nola 19 45
Chris Sale 40 45
Max Scherzer 42 44
Justin Verlander 16 43
Corey Kluber 51 38
Mike Clevinger 18 35
Zack Greinke 28 26
Luis Severino 29 26
Carlos Carrasco 32 23
Clayton Kershaw 39 21
Chad Green 23 18
Kyle Hendricks 24 17
Josh Hader 14 16
J.A. Happ 13 14
Craig Kimbrel 25 14
Yusmeiro Petit 15 14
Raisel Iglesias 17 14
Dellin Betances 12 14
Craig Stammen 11 13
Mike Minor 18 12
James Paxton 23 12
Madison Bumgarner 14 10
Brandon Morrow 12 10

This seems like a good list, for two reasons: (1) it’s intuitive enough to pass the “eye test” in terms of including pitchers like deGrom and Kershaw, but it also includes some “newcomer” aces like Aaron Nola; (2) it includes enough counterintuitive pitchers that we can look into new cases and further define quality pitching roles. Here, I’m thinking of Josh Hader (who is an ace in relief), JA Happ (who has morphed into an extremely consistent late career pitcher), and Dellin Betances (who is typically criticized as a middle reliever when it comes to contract negotiation matters, but should be thought of in the highest terms of consistency). Moreover, this table shows the benefit of being more inclusive in terms of defining aces, rather than less exclusive. Here’s what happens if you only consider pitchers with consecutive 20+ Runs Prevented seasons to be aces:

True Aces? 2017 Average Runs Prevented 2018 Average Runs Prevented
Chris Sale 40 45
Max Scherzer 42 44
Corey Kluber 51 38
Zack Greinke 28 26
Luis Severino 29 26
Carlos Carrasco 32 23
Clayton Kershaw 39 21

This list is visually appealing insofar as we all know that these pitchers are great, the top of the game in fact. This group would be the true 99th percentile of the game of baseball at the moment. But, it does not capture easily acquired pitchers (except for the Cleveland arms, perhaps, in Carlos Carrasco and Corey Kluber, this is an expensive group in terms of draft status, contract, or prospect resources required for acquisition). So, the cases of aces exclude most MLB teams from acquiring aces. Furthermore, there is no diversity of roles in this table, and to my eye that seems like the biggest shortcoming in this definition of the ace: for if we continue to define aces according to the highest possible standard, we will continue to replay and repeat the “Josh Hader should start” debate, and miss the reasons why Wade Miley or Jeremy Jeffress could be aces, too.

Building a pitching staff based around flexible roles, or based on elite relief roles and interchangeable starting rotation roles, does not preclude acehood. In fact, the 2018 Brewers demonstrate that effectively, both with multi-year consistent aces (Hader) and (potentially) one-off successes (Jeffress, Miley).

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