FIP and xFIP

Fielding independent pitching is an alternative-pitching statistic to ERA. Because ERA relies so much on the quality of fielding behind the pitcher, and on the scorekeeper’s objective opinion about what constitutes and error and what does not, it is not the best stat available to evaluate pitching performance. In 1999, Voros McCracken attempted to determine why Greg Maddux’s ERA ballooned from 2.22 in 1998 to 3.57 in 1999. McCracken figured out that ERA is not predictive year in and year out, because batting average on balls in play (BABIP) fluctuate wildly from year to year, even when the pitcher is pitching the same way. Balls that may have been caught the year before may start dunking in for hits the next year, and inevitably some of those base runners will come around to score.

What McCracken determined was that once a ball was put into play, a pitcher no longer had control over what happened to it. Because of this, it is unfair to reward or penalize the pitcher for what happens after the ball is put into play. McCracken developed a formula that credits the pitcher with only the events that he can reasonably influence. FIP is determined by using the formula: (Home Runs x 13 +(walks + hit by pitches-intentional walks) x3-Strikeoutsx2/Innings Pitched + 3.2)

This formula takes away anything that could reasonably be determined to be a factor of luck, and credits or debits the pitcher with only what he could control (walks, strikeouts, home runs). J.C Bradbury sums up McCracken’s findings, “The noise of earned runs generated on balls put in play, which were randomly turned into hits or outs by the fielders, actually hindered the identification of the pitcher’s true ability. It turns out that the real reason Greg Maddux is so good is that, though he is not an overpowering strikeout pitcher, he rarely walks batters or gives up home runs. This makes DIPS (FIP) a valuable tool for disentangling responsibility for preventing runs.”

FIP and Defensive Independent Pitching (DIPS, another similar formula to FIP) fail to account for park factors. A home run over the Green Monster in Fenway Park could be a routine fly ball in 29 other stadiums. Since not every ballpark has the exact same dimensions, it is unwise to use FIP to compare pitchers across different teams, as their home runs allowed may be attributed to their ballpark.

There is, however, a version of FIP that does account for park and league factors.  It is called park adjusted FIP (xFIP).  Because unadjusted FIP fails to account for the varying dimensions of major league ballparks, it is unfair to weight home runs as heavily as it does.  xFIP takes care of this problem by adding a step to the FIP formula.  The formula is: ((Fly balls x.11) x 13 + (walks +HBP –IBB) x 3 –(strikeouts) x 2)/ innings pitched).

By replacing home runs allowed with fly balls allowed (and the rate at which they leave the ball park) xFIP has taken a great stride in park adjusting a pitching statistic. xFIP is a far more accurate depiction of a pitcher’s performance than ERA and should be used accordingly.  FIP and xFIP are more indicative of a pitcher’s actual performance and also more predictive of a pitcher’s future performance.

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