Up until this point, we at Baseball Professor have done a lot of talking about BABIP and its value in determining whether or not a player should maintain a certain level of performance. This is great when analyzing batters, but it's only part of the picture when looking at pitchers.
Today, we’ll examine xFIP.
Before we dive into the potentially overwhelming world of baseball sabermetrics, let’s see exactly what xFIP is. The following definitions are courtesy of the knowledgeable baseball folks from The Hardball Times:
FIP (Fielding Independent Pitching)—A measure of all those things for which a pitcher is specifically responsible. The formula is (HR*13+(BB+HBP-IBB)*3-K*2)/IP, plus a league-specific factor (usually around 3.2) to round out the number to an equivalent ERA number. FIP helps you understand how well a pitcher pitched, regardless of how well his fielders fielded.
xFIP (Expected Fielding Independent Pitching)—This is an experimental stat that adjusts and “normalizes” the home run component. Research has shown that home runs allowed are pretty much a function of flyballs allowed and home park, so xFIP is based on the average number of home runs allowed per outfield fly. Theoretically, this should be a better predictor of a pitcher’s future ERA.
Just like with a hitter’s batting average, a pitcher’s ERA only provides a snapshot of the player’s season. Generalizations can be made, but just because a pitcher had a lower ERA doesn’t necessarily mean he had a better season.
Take the surprising Jair Jurrjens for example. He’s a popular poster-boy for overachievement, and with good reason. Take a look at how his 2009 season compared to his pedestrian 2008:
|IP||ERA||WHIP||Opp BA||Opp BABIP||K/9||BB/9||FIP||xFIP|
Despite lowering his ERA by over a run and his WHIP by 16 points, Jurrjens might have actually had a worse season. His walk rate and strikeout rate remained virtually the same, but his opponents’ BABIP, FIP and xFIP all moved away from the norm. This isn’t to say Jurrjens didn’t make any improvements in 2009 because his FB percentage decreased and his GB/FB ratio was much improved, but clearly his ‘09 season will be extremely tough to replicate.
Now, let’s look at last year’s top 10 in xFIP along with their actual ERA:
|Player||xFIP||ERA||xFIP Rank||ERA Rank|
Note: Ranks are among all 75 qualifying starting pitchers.
First off, what happened to Ricky Nolasco? His GB/FB ratio was almost the same as his successful 2008, but his opponents’ BABIP was well above career averages and his left-on-base percentage (LOB%) was just 61.0 percent. To put that into perspective, it was easily last among those 75 qualifying starters. The next worst (Carl Pavano at 74) had a LOB% of 66.1 percent. Yes, that’s a huge gap.
Also notice how Zack Greinke’s ERA was a whole run lower than his xFIP. This isn’t to say that Greinke isn’t a great pitcher—he’s proven that he is among baseball’s elite—but is he really that much better than Jon Lester? Greinke has slightly better control than Lester, but I see more wins from Boston’s southpaw along with comparable strikeouts and ERA.
So, when evaluating which starters you want to draft, make sure to examine their xFIP. Just remember that it is a defense- and ballpark-independent statistic, and pitchers with sub-par defenses behind them or who pitch in Yankee Stadium, Citizens Bank Ballpark, or Chase Field (to name a few) may not be able to close the gap between their ERA and xFIP (which may explain Lester’s gap as Boston had an unstable defense last season).
Like with any stat, xFIP can be very useful in the right context. Don’t draft your starters without considering it.
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