Advanced Stats 102: FIP and xFIP
FIP tells you how a pitcher should have performed, not how he actually performed. Confused? Let me explain. A pitcher can only control certain aspects of a game: such as walks, hit batters, wild pitches, etc. What FIP does is takes away the team performance behind the pitcher, and tells us what the pitcher’s ERA should have looked like over a period of time without using defensive statistics. So basically, FIP takes what a pitcher can control, walks, home runs, etc and predicts that players true talent level. One thing to remember in FIP is that the larger the sample size, the more accurate the stat becomes.
The formula for FIP is: ((13 x HR) + (3 x (BB + HBP – IBB)) – (2 x K)) / IP + constant
* The constant is merely a number in order to place FIP on the same scale as ERA. It is usually between 3.15- 3.20.
So in a nutshell, FIP takes away the defense’s impact on a pitcher and tells us the story of the pitcher’s performance based only on what the pitcher has control over. It we reverse the nomenclature to “Pitching Independent of Fielding”, that may help explain it better. For example:
xFIP (Expected Field Independent Pitching)
xFIP is almost the same as FIP, except that xFIP factors in how many home runs the player should have allowed. xFIP replaces the number of home runs a pitcher gave up with an estimate of how many they are expected to surrender. This one is a little more difficult to understand, because it uses HR/FB% (home run to fly ball).
Here is the formula: xFIP = ((13*(FB% * League-average HR/FB rate))+(3*(BB+HBP))-(2*K))/IP + constant
We’ll use Matt Harrison again as our example:
So, in summation, both FIP and xFIP are studies of a pitcher’s own ability and takes away any help from his defense. It’s a measure of “true ERA”, based only on what that player can control, like K’s, BB’s, walks and hit batters.There are those that do not like xFIP, because it is a prediction based on fly ball rates. However, it is still valuable, since it factors in the league average. Maybe the most important thing to remember about FIP and xFIP, is that the bigger the sample size, the more accurate the stat is.