Fielding Independent Pitching, or FIP, is a metric designed to measure only what is in the pitchers control. Scaled to look like ERA, FIP takes into account only strikeouts, walks and home runs, the three true outcomes that are not heavily influenced by the defense behind a pitcher, and attempts to show what a pitcher’s ERA “should” look like were it not for the effects of luck and defense.
The reason for this kind of measure is that, through research and data, we know how little of an influence a pitcher has on what happens to balls in play. While some do exert influence on the kind of contact hitters make, it is minimal compared to that of luck, defense, and sequencing. A pitcher can’t control whether a blooper falls in, or a line drive is caught, or very much in between, and FIP accounts for that.
And while some pitchers can and do consistently run ERAs lower than their FIP, in most cases, FIP will do a better job of telling you how the pitcher himself actually performed, regardless of what his defense did or didn’t do. Fangraphs agree’s with this, and uses FIP in their WAR calculation, while Baseball Reference uses a runs-allowed based formula. That means that there isn’t a complete consensus between sabermetricians as to which pitching metric is better at measuring value, though most tend to lean towards FIP.
The formula for FIP is as follows:
FIP = ((13*HR)+(3*(BB+HBP))-(2*K))/IP + constant
As you can see, it is a fairly simple and understandable formula. The only part you may be unfamiliar with is the constant, and that is simply used to convert it to our traditional ERA scale (3.00 is great, 4.oo is average, etc.).
FIP is essentially telling us how well the pitcher controlled what he is meant to control. Whether or not he got hitters to swing and miss, thus avoiding balls in play that could turn into hits, whether or not he threw strikes and didn’t allow free base runners, and whether or not he gave up a home run.
However, home runs can be subject to noise as well. Generally, pitchers will fall extremely close to the mean when it comes to Home Runs per Fly Ball (HR/FB). More often than not, how often a fly ball leaves a yard is not dependent on who gave up the fly ball. So, if we see a pitcher with an extreme HR/FB rate, either high or low, in a smaller sample size, it is safe to assume that there are factors at work that are beyond their control.
Expected FIP (xFIP) can help in that situation. It is almost exactly the same as regular FIP, except that it uses a home run total derived from a pitcher’s fly ball rate and league average HR/FB rate rather than just taking the raw home run total.
xFIP = ((13*(Fly balls * lgHR/FB%))+(3*(BB+HBP))-(2*K))/IP + constant
So as you can see, the only difference in the formula is that 13*HR is replaced by 13*(FB * lgHR/FB), which is the change I outlined before. We are attempting to rid ourselves of any HR/FB noise that could be present.
There are some things to note with xFIP, however. There are rare situations – particularly if the sample size is large – in which a pitcher has “earned” an extreme HR/FB rate. If that is the case, xFIP could be overrating (or underrating) said pitcher. The best way to determine something like this is to look at their career HR/FB rate. If it is consistently extreme, year in and year out, the pitcher is likely influencing it to a degree. For example, if a pitcher has a 6% HR/FB rate over a five year career, they probably deserve credit for that.
Neither FIP nor xFIP are perfect, and should always be used in conjunction with each other, and any other relevant information at your disposal. ERA, FIP and xFIP together should usually give you a nice idea of how a pitcher performed, as they can work together with various checks and balances, but you should always dig deeper if you want a more detailed view.