What Is x ERA?

Key Metrics For Baseball Betting


With baseball season having arrived, TheLines’ has prepped some primers on key metrics that can aid your daily handicapping. We already covered FIP in the first edition, and now we’ll go over a very similar metric: xERA.

So, what is xERA and how can you apply it when betting on MLB?

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What Is xERA?

xERA operates in much the same way as FIP. Like FIP, it uses the ERA scale but tries to improve upon ERA by more accurately ascribing the stat to things over which the pitcher has control. Much like FIP, xERA starts with strikeouts, walks and hits-by-pitch.

The big difference with xERA comes in how it treats balls in play. Where FIP ignores them and simply charges the pitcher with home runs, xERA uses Statcast data (launch angle, exit velocity, sprint speed) to estimate the amount of damage the pitcher is allowing on batted balls. In fact, xERA simply translates the batting stat xwOBA (Expected Weighted On-Base Average) to the pitcher side and transforms it to the ERA scale.

FIP and other fielding-independent metrics have been around since the early 2000s, but xERA only became a possibility with the advent of Statcast tracking data, which began going public around 2015. It gave fielding-independent pitching evaluation a dimension it simply never had before.

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Why Is xERA Useful?

Much like FIP, xERA gives us a superior look at how a pitcher has performed. ERA contains a large amount of noise and depends heavily on luck and the defense behind the pitcher.

xERA strips these factors out and simply looks at true outcomes (walks and strikeouts) and the amount of damage similarly hit baseballs have caused on average. So, allowing a hard shot to the deep part of cavernous Oracle Park that winds up caught would count against the pitcher, since that would frequently result in home runs in most instances. Similarly, a slow seeing eye grounder that brings home a run would ding the pitcher’s ERA but not count for much against his xERA.

Statistics and analysis site FanGraphs is probably the best and easiest place to find xERA, listed right next to ERA for an easy comparison. If you want to dive a little deeper into the Statcast stuff that drives xERA, head over to Baseball Savant. They have player pages that go into fascinating depth on batted ball stuff, spin rates, etc. with percentile rankings that put everything into context.

Using xERA: The Case Of Julio Urias

To see how xERA can give us a more complete picture of a player, let’s take a look at Dodgers starter Julio Urias.

Heading over to his FanGraphs profile, we can see that he has a career ERA of 3.09. Yet other ERA-scale metrics FIP/xFIP/SIERA sit at 3.32/4.09/4.02. When the sample size is north of 400 innings, it seems likely these metrics are missing something about Urias.

That something is his contact management. Urias has consistently induced weakly hit baseballs over his career, which we can see in his average exit velocity of 85.9. Using last season’s Baseball Savant leaderboard, we can see that number would have ranked fourth among qualified pitchers. As it was, Urias actually ranked … fourth at 86 mph. That’s clearly no fluke.

Furthermore, Urias specifically induces a large number of weakly hit fly balls. He ranked eighth in average launch angle allowed last season at 15.5. While he didn’t allow quite the number of infield flies as usual last year (9.9% per FanGraphs), he had titanic numbers of 13.9% and 15% in the two prior seasons.

Metrics that don’t account for Urias’ talent for inducing weak contact are going to underrate him. That’s where xERA more clearly reflects his ability.

Use xERA To Help Judge Pitchers’ Talent Levels

Statcast has given us an incredible wealth of new data. Statisticians are using this data to create metrics that more fully capture what’s happening on the field, and xERA serves as a prime example. Never before have we had an all-in-one pitching metric that combines fielding-independent stuff with batted ball information.

ERA is not a reliable stat for judging pitchers’ talent levels because it includes a large amount of noise and variance.

Use xERA to help filter some of that out and get a better look at the quality of the pitchers in a baseball game. Make sure elite contact managers like Urias are being priced appropriately, and if not, look for value spots to back them.

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