Zach Britton’s 2016: An All-Time Great Season?

It seems as though every year we ask the question “Is Player X having the greatest season by a relief pitcher in history?” Craig Kimbrel’s 2012 performance raised the question, as did Wade Davis’ performances both in 2014 and 2015. This year, the guy putting up a season which threatens the record books is Orioles closer Zach Britton.

On April 30th against the White Sox, Britton took the mound in the ninth inning of a tie game. He struck out the first two batters he faced and then twisted his ankle trying to field what ended up being a bunt single for Adam Eaton. Britton had to leave the game due to the injury and the Orioles brought in Vance Worley to replace him. Worley promptly walked a batter before giving up a single which scored Eaton and ultimately lost the game for the Orioles. That was the last earned run Zach Britton allowed. Go ahead and read the first three words of this paragraph again… that’s right, the last earned run Zach Britton allowed was on April 30th.

Three days before that outing against Chicago, Britton yielded a two-out run on an RBI single which was only made possible when the runner on first was able to advance to second via defensive indifference — which itself was only made possible by the Orioles’ three-run lead. To find an earned run that’s clearly attributable to Britton and not injury or strategic circumstances beyond his control, you have to go all the way back to April 11th, his fourth game of the season, when he allowed a home run to Mookie Betts to lead off the ninth.

And there, in two paragraphs, we have relived every single earned run Zach Britton has allowed during the 2016 season — all three of them. Last night, he made his 38th appearance since allowing his last earned run, which moved him into a tie with Brett Cecil and Craig Kimbrel for the longest streak in the Baseball-Reference Play Index era (since 1913). One more outing without allowing an earned run and the record will be all Britton’s.

Due to this absurd streak, we now find ourselves in a world where Zach Britton is out there with an almost unfathomably low 0.57 ERA in the second week of August. If that sounds historically great, well, it certainly would be. Here’s the all-time season ERA leaderboard for pitchers with a minimum of 60 innings pitched:

If Britton were to somehow sustain his 0.57 ERA over the rest of the season, it would be the best season-long mark in history. Of course, we know that ERA isn’t the best measure of a player’s season — or even a particularly good one, for that matter. The “earned” portion of earned runs is a trivial distinction reliant on trivial rules and the whims of official scorekeepers. I am partial, however, to evaluating a pitcher’s season in large part on their success in accomplishing their primary objective of run prevention, even though much of that success or lack thereof can be attributed to luck and/or defensive performance. There are a plethora of stats we can consult which are notably more predictive and better isolate a pitcher’s true performance level. Those stats are invaluable for in-depth analysis, but as for who actually had the best season? Run prevention is the first place I’m looking.

With that in mind, I turned to RA9 (runs allowed per nine innings), which groups in all runs, earned and unearned. Britton has allowed three unearned runs this year – all of which came in the same game – bringing his season total to six runs allowed and giving him a stellar 1.15 RA9. Using the Play Index again, I built a leaderboard of the best seasons by RA9:

What Britton is doing this season is phenomenal, but not entirely unprecedented. His 1.15 RA9 would currently slot in right between 1990 Eckersley and 2010 Kuo (remember him?!). It’s worth noting that he’s currently on pace to throw around 68 total innings this season and, were he to somehow reach that innings total without allowing another run, his RA9 would fall to a minuscule 0.79. Just for fun, here’s a table showing what Britton’s end-of-season ERA and RA9 will look like as a function of his future runs allowed assuming a final season mark of 68 innings:

From a pure run-prevention standpoint, this leaves the door open for the possibility that Britton could force himself into the “best relief season ever” conversation if he manages to yield one run or fewer through the end of the season. Of course, maintaining his current torrid pace is much easier said than done.

Britton has built upon the extraordinarily successful formula he’s developed in recent years: destroying opponents with an entirely unfair mid-90s sinker. He’s using the pitch more than 90% of the time for the third consecutive season and the whiff and ground-ball rates illustrate why. According to the Baseball Prospectus PITCHf/x leaderboards, Britton’s sinker is generating a league-leading 40.3% whiff/swing rate — Jeurys Familia’s sinker is a distant second at 28.3% whiff/swing — while the ground-ball rate on the pitch also leads the league at 80%. Of course he throws the pitch almost exclusively: why would he ever need to throw anything else? It’s a beautifully simplistic and effective approach: “Here’s my sinker, on the off-chance you can hit it, good luck getting it in the air.”

The biggest change for Britton this year has been the rate at which he’s inducing whiffs. That 40.3% whiff/swing rate on his sinker represents a significant jump from the (still league-leading) 30.9% rate he posted a year ago. Likely causes of this jump include the fact that the velocity on the pitch has increased about half a tick this season and he’s commanding it to the bottom of the zone even better than he did a year ago. On the left in the graph below is the 2015 heat map of his sinker and the 2016 heat map is on the right:

Britton was one of the best pitchers in the game last season and, if anything, he’s even better today than he was then. However, his stats are also buoyed by the fact that opponents have posted an unsustainably low .110 BABIP on ground balls against him this season, well below the league-average .243 BABIP on grounders. Although it might be reasonable to suppose that, as a ground-ball specialist, the weakness of the contact he induces could lend itself to atypically low BABIPs, recent history doesn’t support that theory. In 2015, his BABIP on ground balls was a pedestrian .266. Britton is an undeniably great pitcher right now, but that doesn’t negate the fact that he’s also benefited from a certain amount of good fortune.

Anything can happen over the roughly 20 innings remaining in Britton’s 2016 season. A few bad breaks or days when his sinker isn’t sinking could cause his stat lines to fall back towards the pack and merely appear fantastic rather than extraordinary. Then again, perhaps this current extraordinary run of success continues and we’re currently watching one of the best seasons of run-prevention we’ve ever seen.