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.
Click here for a link to the full interactive chart.
Below is a graphical depiction of all qualified starting pitchers from 2019 and their ERA (bars) vs xFIP (black target lines)
Click here for a link to the full interactive chart
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