Defining the 'Ace' label with statistics

Like so many other aspects of baseball, the criteria used in the attempt to define an ace starting pitcher often resorts to the answer being in the eye of the beholder, or at the least, "I know it when I see it" territory. Superlatives are thrown about. Heads shake in disagreement. With no legitimate, tangible definition met, how could we possibly separate an ace from any other starter -- aside from the slot in their given rotation?

Let's try to put an end to this, at least for this season. Using statistical data available as of July 29th, incorporating the more common descriptors of the ideal ace and creating a well-defined set of criteria of what makes a starter exceptional, we can finally come to somewhat of a consensus as to what this occasionally imaginary hurler really looks like.

When the idea of who is an ace is discussed, there is an immediate divergence; those who feel that every team has one -- their No. 1 or Opening Day starter -- and those who feel that the title of ace can only go to a fortunate few. Personally fitting into this latter category, I went to work crafting my criteria and came up with a list of desirable qualities that will give us a reasonable answer steeped in fact.

The first order of business was to go to FanGraphs, do a quick query and export of a number of statistics for qualified starting pitchers for 2013. In the end, the query gave us 89 starters. With respect to what defined someone as qualified, it essentially broke down to starters who had thrown at least 100 innings over at least 17 games started.

With this first step complete, let's get to work and do some addition through subtraction with the help of some definitions that describe an ace.

Plus Pitches

It is generally accepted that the most successful starters will have at least three quality "plus" pitches that they can use at just about any time to get a hitter out. On a more subjective slant, these should be "nasty;" superior velocity, break, movement or location can make it nasty, it really doesn't matter.

How do we measure plus pitches, and therefore nastiness?

Pitch linear weights will do the trick here -- in particular, the PITCHf/x Pitch Value per 100 statistic. Briefly, it provides a value to a pitcher's arsenal through run expectancy changes with each pitch. For our exercise here, we want to find guys with three or more pitches that have a positive pitch value. While it does have its caveats, it does provide a reasonable and defensible measure of the ever-elusive "stuff" a pitcher has. After applying a quick formula to our data, we arrive at our first separation of the wheat from the chaff; out of the original 89 starters, we now are down to 54 with at least three plus pitches. In fact, of the remaining 54, we have 30 pitchers with three plus pitches, 18 with four plus pitches, and a very special group of six hurlers with five plus pitches as determined by PITCHf/x pitch values:

Name	Team
A.J. Burnett	Pirates
Matt Harvey	Mets
John Lackey	Red Sox
Justin Masterson	Indians
Chris Sale	White Sox
Max Scherzer	Tigers

While this is a great start, we are still in need of further paring down towards our goal of defining ace-ness, and while plus stuff and nasty pitches will get you far, they don't necessarily equate to a superior starter. Therefore we trudge onward, one step closer to the answer ...

FIPness

Another quality fans and scouts alike discuss in wanting in an ace starter is the ability to control the game and be responsible for the vast majority of the run suppression against the opponent. Using some of the components of Fielding Independent Pitching, we can get a great idea of how a potential ace can do exactly this. Through the concept of FIP, we can make the assumption that an ace will maximize strikeouts while at the same time minimizing walks and homeruns. I would also include the caveat that aces also induce a greater number of swinging strikes than his non-ace counterparts, which ties in nicely with our first criteria of an ace having three plus pitches.

With that in mind, let's take four stats: strikeout rate (K%), walk rate (BB%), homeruns per flyball rate (HR/FB%), and also swinging strike rate (SwStr%), and compare our remaining 54 hurlers to one another in these categories. We will use the 2013 MLB averages for each stat to define our cutoff points, which are are listed here:

K%	BB%	HR/FB%	SwStr%
>18.8	<7.4	<10.7	>8.6

After applying these criteria, we are down to 12 pitchers:

Name	Team
Madison Bumgarner	Giants
Patrick Corbin	Diamondbacks
Matt Harvey	Mets
Jeremy Hellickson	Rays
Felix Hernandez	Mariners
Derek Holland	Rangers
Clayton Kershaw	Dodgers
Mat Latos	Reds
Mike Minor	Braves
Max Scherzer	Tigers
James Shields	Royals
Adam Wainwright	Cardinals

... and an impressive 12 they are. It's a nice mix of some top of the rotation stalwarts along with some youngsters that are just now coming into their own.

But we're not done yet. Plus stuff, an ability to control the scoring environment almost single-handedly in the form of maximization of the tenets of fielding independent pitching, and now ...

Going the Distance

Durability and the ability to be a workhorse for the rotation while minimizing the use of relievers is another highly desired trait that ace starters provide. The unwritten rule of a starter's durability is that a starter should be able provide 200+ innings per season. Since no one is even close to coming to 200 innings pitched as of yet in 2013, we will need to use an alternate marker of durability for our group of twelve aces-to-be. For our exercise, we will use something I'm calling "expected innings pitched" or xIP. The expectation, in this day and age of closers and setup men, is that a starter's desired workload would be seven innings per start. So with this in mind, we come to our next criteria:

IP - xIP ≥ 0

... where xIP = games started * 7

So who moves on? Who can we consider to be the aces of 2013?

Clayton Kershaw and Adam Wainwright.

Here are their final stats:

Name	Team	GS	xIP	IP	IP_diff	IP/GS	HR/FB	K%	BB%	SwStr%
Adam Wainwright	Cardinals	22	154	161.67	7.67	7.35	6.00%	22.9%	2.8%	10.3%
Clayton Kershaw	Dodgers	22	154	160.33	6.33	7.29	6.50%	25.5%	5.7%	11.7%

...and for those curious, their pitch values:

Name	Team	wFA/C	wFT/C	wFC/C	wSI/C	wSL/C	wCU/C	wCH/C
Adam Wainwright	Cardinals	2.28		1.87	-0.65		2.16	-3.14
Clayton Kershaw	Dodgers	0.95	0.67			1.22		1.86

With such a tight set of criteria, there are a few starters who didn't quite make the cut -- but came awfully close -- and should be given an honorable mention.

Seattle's Felix Hernandez provided all of the aforementioned criteria, but fell short in the xIP criteria ... by one batter. Lefty hurlers Chris Sale (10.7%) and Cliff Lee (11.6%) also deserve recognition for their stellar performances, albeit ones that just failed to meet the HR/FB rate cutoff.

There we have it -- with some thoughtful manipulation of the available data to go along with a couple of rational assumptions made regarding how to assign an objective value to some fairly subjective descriptions of what makes an ace the ace he is, we have our aces of 2013.

... and we did it without one mention of wins. Brian Kenny would be so proud.

. . .

All statistics courtesy of FanGraphs.

Stuart Wallace is a writer at Beyond The Box Score. You can follow him on Twitter at @TClippardsSpecs.