My ERA Academia: Are the Red Sox’s Left-Handed Relievers Overperforming?

Most of your traditional baseball statistics are descriptive. They tell you what has already happened. Earned run average (ERA) shows how many runs a pitcher allows across nine innings. ERA tends to ruffle some because it fails to consider factors outside of a pitcher’s control, like team defense. A pitcher with a low ERA could be blessed with a strong defense behind him, whereas a pitcher with a high ERA might not be getting bailed out by luck or good gloves.

Fielding independent pitching (FIP) was created to isolate a pitcher’s performance. It measures events that pitchers have control over — strikeouts, walks, hit-by-pitches, and home runs — and removes outcomes outside a pitcher’s control. Expected statistics attempt to measure what “should” have happened. Anything with an “x” before it is an expected stat. xERA converts xwOBA to the ERA scale. xwOBA factors how much contact (hits, strikeouts, walks, and hit-by-pitches) and the quality of contact (launch angle, exit velocity, and hard-hit rate) to determine what a player’s wOBA should’ve been. xFIP uses the same equation as FIP, but swaps out the actual number of home runs allowed for the number of fly balls multiplied by the league average home-run-to-fly-ball rate. Expected statistics are not predictive by any means, but including them in our analysis helps us paint a clearer picture of how effective a pitcher was at preventing runs, separate from what happened around them.

Among left-handed relievers, the Red Sox have posted a 2.01 ERA, the lowest in the majors. Their 3.38 FIP ranks tenth. Thanks to their strong defense, more balls in play are being converted to outs than expected. Willson Contreras has been phenomenal at first (3 DRS), and Caleb Durbin is one of the best defensive third basemen in the American League.

Boston’s lefty relievers' -1.35 ERA-xERA differential is the lowest in baseball, meaning their ERA is outpacing their expected ERA (3.38) by a considerable margin. Looking ahead, there’s bound to be some regression for the group. Aroldis Chapman (-2.58) and Tyler Samaniego (-3.85) hold the two highest ERA-xERA differentials. Last year, Chapman outperformed his expected stats, and unlike Samaniego, he’s posted the 7th highest strikeout-minus-walk rate in the American League this year. I’m more concerned about Chapman’s drop in velocity, which has been a glaring issue since Opening Day.

Jovani Morán has looked sharp, though I feel bad that he’s being charged for runs as the sacrificial lamb opener during Brayan Bello’s starts. Danny Coulombe dealt with an injury (cervical spasms) earlier this year. His 2.20% K-BB% is concerning, and he hasn’t shown an ability to strike guys out or induce ground balls. The team’s minus -0.59 FIP-xFIP differential isn’t as substantial, and the team’s defense has been one of the bright spots in an otherwise not-so-great season.

I just threw a ton of numbers at you. (I can’t help it; I’m a mathlete!) What does this all mean? So far, the Red Sox’s left-handed relievers have been outperforming, and they’re bound for some regression. How much is the real question.

Expected stats are fun, but no single equation should carry all the weight in an analysis. Despite Trevor Story’s injury, the team’s infield defense is stronger without him, and I think that it will hold up better than expected. Sox fans have witnessed a revolving door of relievers toss in games, but among the qualified players, lefties only account for three of the seven pitchers, and Justin Slaten’s return bolsters the bullpen. Even if one of the lefties regresses, he should help them pick up the slack.