Sabermetricians are generally viewed as experts on statistics, so go find your favorite sabermetrician and ask him the ERA of Pitcher X in the MLB. Chances are, he might not even know what it is. Now ask the same sabermetrician Player X's xFIP and he probably has a much clearer idea. So what is xFIP and why do statisticians prefer it over a more widely accepted statistic like ERA?
xFIP, or expected fielding independent pitching, estimates a pitcher's performance based solely on events within his control - strikeouts, walks, hit-by-pitches, and flyballs allowed. FIP, or fielding independent pitching, attempts to do a similar calculation, but xFIP goes a step further by lowering a pitcher's HR/FB rate to the league average due to the general year-to-year volatility in the HR/FB metric. The scale of xFIP is similar to ERA, so many people (mistakenly) use the two interchangeably and often completely replace ERA with xFIP as a way to better understand a pitcher's performance without other factors like defense, luck, or sequencing.
The problem with relying on xFIP as the sole measurement of a pitcher's performance is similar to the flaws in using BABIP as the sole measurement of a hitter. Despite the high variability in pitcher's HR/FB rates each year, most pitchers still have a certain level of control over that metric. That means that immediately setting HR/FB rate back to league average can and will understate or overstate a pitcher's skills, depending on the pitcher's level of control over his home run rates.
As a result, there are several statistics that are useful to see alongside ERA vs xFIP to better understand how "earned" his HR/FB rate actually was. These stats include Brls/PA%, park factors and Hard%, while looking at statistics like BABIP and LOB% in conjunction with the others also helps better paint a picture of how "earned" a pitcher's ERA actually was.
Below is a dashboard sorted by the pitchers with the largest disparities between his ERA and xFIP.
Below is a graphical depiction of the xFIP/ERA delta between all qualified starting pitchers from 2019 and their NFBC ADP from the 12-team Online Championship.
Below is a graphical depiction of all qualified starting pitchers from 2019 and their ERA (bars) vs xFIP (black target lines)
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Our player projections model was created and is maintained by our founder and lead Statistician Anthony Perri. Using advanced and cutting edge Sabermetric indicators, which now include velocity indicators on batted balls, our player projections are trusted for accuracy by several Major League Baseball front offices.