I join many others in welcoming Bill Simmons to the statistical revolution in baseball. While statistics have always been an integral part of baseball, we have learned to tinker with them to more accurately reflect what happens on the field, and more importantly, what will happen. One of the more difficult puzzles to crack has been evaluating pitchers due to how entangled their performance is with that of their defenders. Accepting the move from ERA to Fielding Independent Pitching is probably the single biggest step one can take on the right path of separating the two, and Simmons has made that leap.
The Sports Guy made the case for FIP in his piece by referencing White Sox closer Bobby Jenks, who posted a decent 3.71 ERA last year, but whose secondary stats added up to a more mediocre 4.47 FIP. Given that Jenks’ ERA was significantly lower than his FIP, he concluded that Jenks wasn’t as good as his traditional numbers made him appear. While this is usually true, and the process he used to make his conclusion works most of the time, there is one more important number to check.
Home Runs allowed, an important input to the FIP formula, are not as skill-based as had been thought throughout history. Research has shown that pitchers have little control over how often a fly ball actually leaves the yard. In fact, if you want to predict how many home runs a pitcher will give up in 2010 you are better off looking at his 2009 ground ball ratio rather than his 2009 home run totals, the latter of which can occasionally include some good or bad luck due to wind, park, or random variation.
That is why xFIP exists, to correct those home run rates. It is simply the FIP formula with an expected home run rate based on fly
ball totals, rather than actual home run rate. Substituting for the average amount of fly balls that turn into home runs leads to more accurate future projections than just looking at FIP by itself. Sticking with Simmons’ example of Bobby Jenks, he certainly did look like garbage at times last season, in large part because 17 percent of his fly balls went for home runs. That’s nearly double his career rate and well above the league average, which is around 11 percent. While Jenks’ ERA of 3.71 didn’t match up well with his 4.47 FIP, his xFIP of 3.63 shows that Jenks had some bad luck on fly balls clearing the wall.
Bobby Jenks rediscovered his strikeout touch in 2009 (8.3 K/9) and kept his walks low (2.7 BB/9). If his home runs hadn’t ballooned, he would have been seen as one of baseball’s best closers. While Bill was right to look at FIP to see whether Jenks’ ERA reflected how he really pitched, making that last small step to using xFIP to project his future performance will give him, and you, an even greater advantage.
Matthew Carruth is an author of FanGraphs.