LA Angels Monday News Crash:

FIP just stands for “fielding independent pitching” – it is an ERA alternative (ie, it is scaled to ERA) that just looks at what is overwhelmingly under a pitcher’s control: strikeouts, walks, hit batters, homeruns. It’s called a “true talent” metric for that reason, and there’s some correlation between FIP and future ERA over large samples.

It assumes that once a ball leaves the bat, much is down to luck or defense, so this metric subtracts all that, and just looks at the basic things a pitcher controls. If there’s a big delta between ERA and FIP, it may be due in part to team defense (eg, the Angels have one of the worst defenses in the MLB behind Detmers).

Of course, it’s not that simple – some pitchers who pitch to contact induce weak contact, pop-ups, etc, all of which is its own skill, and FIP will tend to always favor strikeout artists who keep the ball in the park and don’t give up many free passes – and disfavor groundball pitchers (like Koch) who pitch to defense.

ERA+ is just a way of looking at ERA in a park and league context – where a mark of 100 means league average performance adjusting for park contexts (so as not to penalize Coors pitchers, or overly favor San Diego pitchers, etc).

FIP is useful, but incomplete. If you want to look at a couple similar metrics that also take into account batted ball data, trying to give some credit to pitchers who induce weak contact, there’s xERA (expected ERA) and SIERA. SIERA gives credit for inducing ground balls and pop-ups, and xERA is even more sophisticated, as it uses Statcast data to estimate a pitcher’s performance based on the quality of contact allowed—exit velocity, launch angle, and runner speed—rather than just the outcomes.

I like xERA for that reason, and look at it in combination with FIP/xFIP to assess (directionally) whether a pitcher has been the victim of bad luck and poor defense relative to ERA (which among other things, also penalizes a pitcher for bad bullpens as well).