So just how great (or not great) is Joe Kelly's stuff, exactly?

It's the joke that keeps on giving. "Joe Kelly has Great Stuff" has been an oft-used turn of phrase to jab at the young right-hander's struggles. There have been times when Kelly dazzles and the joke becomes far less of a jest, and there have been times when that very same great stuff resulted in Chernobyl-scale meltdowns happening right before our eyes on national television. Kelly had prior success with the Cardinals before joining the Red Sox and indeed improved down the stretch. As a product of the St. Louis pitcher assembly line, he's got the pedigree of a pitcher with expectations.

Therefore when Kelly stated that he would win the 2015 AL Cy Young award and Curt Schilling lauded his "Great Stuff" on the air, his struggles naturally spawned many a joke. However, Kelly has succeeded before and pitched to a 3.77 ERA in the second half. Is there truth to the claim that his Stuff is indeed Great?

More importantly, what is Great Stuff? There isn't exactly a one-size-fits-all indicator for the Greatness of Stuff. Strikeouts and walks, and the ratio of strikeouts to walks, could be the key. However, control artists don't necessarily have Great Stuff; they can just put their arrow-straight fastballs and junky off-speed pitches wherever they damn well please. When we think of Great Stuff, we think of Clayton Kershaw's curveball and Carlos Carrasco's gun locker full of ammunition. We think of Wade Davis and Dellin Betances blowing fastballs by hapless batters before unleashing a loop-de-loop hammer that allows us to look into some poor sap's eyes and watch his soul break into fifteen small interlocking pieces. Great Stuff is like pornography. You can't define it, but you certainly know it when you see it, and it's often described as being filthy.

This brings us back to the matter of Joe Kelly. His walk and strikeout rates were both worse than league average, and his WHIP was an unsightly 1.44. As we said before, however, Great Stuff and great control don't necessarily go hand-in-hand. A maestro's slider is useless if the pitcher can't put the ball where he wants it. A laser beam fastball may as well be fired from a blunderbuss if it can't be wrangled into the zone. Plenty of pitchers are blessed with Great Stuff. Not all of them can corral it.

A brief disclaimer is in order before we dive in. I am not a scout. I cannot look at any pitcher from behind the backstop and pin a 20-80 grade on a pitch and say that I've made an accurate adjustment. I do not watch Joe Kelly pitch on a regular basis, and I am not an expert at, well, anything. I've managed to get you to read this far, though, so stay where you are for just a moment.

To do a Beyond the Box Score version of Mythbusters and ascertain the quality of Kelly's Stuff, we'll use the PITCHf/x numbers over at FanGraphs. FanGraphs allows one to finagle with leaderboards of pitch movement averages, pitch values, and more. Kelly throws five pitches (a four-seam fastball, a two seamer, a slider, a changeup, and a curveball). Only the two seamer (0.29) and changeup (1.03) registered as being worth a positive number of runs per 100 pitches in 2015. Both of those scores are much better than the league average marks of -0.19 and 0.04, respectively. However, the other three pitches were below league average by a fair amount.

As with many statistics in baseball, numbers can be deceiving. Kelly pitched to a lower ERA in the second half, so we assume that positive regression occurred. Yet batters hit for a higher wOBA against Kelly in the second half than they did in the first. Baseball stats are fickle mistresses that are not to be trusted under any circumstances unless the requisite twenty linear weights have been applied. And they're probably still decently inaccurate in that instance as well.

These numbers aren't fully accurate measures of the quality of Kelly's Stuff, or any pitcher's Stuff for that matter. They're merely the results of what happened when Kelly decided to throw that pitch. End results aren't accurate measures of Stuff. There isn't a good overarching statistic to get an indicator of the quality of a pitch in a vacuum. Certain pitches gain and lose value in different situations. An inside breaking ball could be a great out pitch in a 1-2 count, but you don't necessarily want to throw it 3-0.

To truly get an accurate statistical measurement of Stuff, one may need to factor in things like the receiver's framing aptitude, how well the pitcher hits the target provided by the catcher, pitch velocity and movement, the average size of the strike zone that the umpire calls, how deep a pitcher is into his outing relative to the average length of his outing, temperature, altitude, and possibly the count. That's not only wildly above my pay grade and ability, but that may in fact rely on factors that aren't fully measurable* yet (accuracy of the pitch relative to where the catcher sets up).

*Editor's note - Command is measured by Command F/X, which apparently has mixed reviews on its effectiveness

In lieu of secretly being a statistical genius, I present the following tables. The first compares the league average horizontal movement on the pitches that Kelly throws, and the second does the same with vertical movement. The measurements are in inches. A negative value doesn't indicate poor movement, but rather the direction of the movement. For a righty like Kelly, a positive number in horizontal movement indicates arm-side run, while a negative number means glove-side run. For the vertical numbers, the positive and negative numbers refer to rise and sink.

Well this is fascinating. Kelly's pitches have above-average run in every category, sometimes by quite a bit. The vertical data is a different story. His curveball drops more than the average hammer, which is a very good thing. The four-seam fastball and changeup are heavier than most, and the two-seamer has a bit more altitude.

What's truly interesting here is his slider number. The pitch seems to move in the opposite direction compared to how the league at large throws it. It's got more run than the average slider, and it drops like an elevator. The league as a whole seems to have some trouble with getting their sliders to dramatically break downwards. Kelly's has some serious hook in it. How did batters fare against it?

Oh. Well. That's sub-optimal, to say the least. Opposing batters knocked the slider around to the tune of a wRC+ of 140. That's what happens when a slider gets left out over the plate. Kelly has trouble locating the pitch, perhaps due to the dramatic movement that it possess. The curveball, which we've shown also moves around quite a lot, was hit even worse, to the tune of a 168 wRC+. The only one of his secondary offerings to be effective was the changeup. It produced an outstanding 32 wRC+.

So does Joe Kelly have Great Stuff? It's possible that he does. We've shown that his pitches have way more horizontal movement than the average pitcher's. However, it seems that he hasn't been able to harness that movement and use it for his evil purposes and let slip the dogs of war on the batters of the AL East. As I stated before, a pitcher's Stuff is only as good as his placement of his pitches. Kelly hasn't mastered that art just yet. It's entirely possible that he does. But there's a part of me that wants to say his future may lie in the bullpen, where he can reach back for more velocity and be deployed as a multi-inning weapon. As the saying goes; location, location, location.

Does Joe Kelly have Great Stuff? Probably, yeah. Stuff isn't Joe Kelly's problem, though, and that's what makes him so frustrating.

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