The Relationship Between FIP and Exit Velocity
One of the great things about FIP, in my estimation, is the ease with which one can understand its value. If you’re watching a pitcher for your preferred team and ask yourself “What outcome would bother me the most right here?” a home run is the clear answer. A walk is second. A single, double, or triple isn’t ideal, of course. In the case of every ball in play, though, there’s at least some chance for the defense to make a play. The walk and home run don’t allow that. They are, almost uniformly, decisively negative.
Conversely, a strikeout is generally the best outcome for a pitcher*. A batter who strikes out create no opportunity for value.
*Outside of a double play, of course. That requires a runner at first and less than two outs, though, something that happens less than 20% of the time.
What FIP does is to take those three outcomes and transform them into a pitching stat that’s consistent from year to year and better predicts future ERA than ERA itself does. One thing for which FIP doesn’t account, though, is all of those balls that are hit into play.
Or maybe it does.
We know that a pitcher exerts a decent amount of control over the types of batted balls he concedes. He might be a ground-ball pitcher, a fly-ball pitcher or a mix of both. Newer data pushes us closer to the conclusion that a pitcher has some control over how hard a ball is hit, as well — although most of the control does appear to come from the batter.
Statcast has given us the ability to help reach those conclusions. The graph below comes from the work of Sean Dolinar and Jonah Epstein — you can play around with their tool here — and illustrates the degree to which a pitcher’s observed launch angle and exit velocity represents his true-talent launch angle and exit velocity.
As you can see, there’s more hope for arriving at something like “true-talent” launch angle. And this makes sense: as noted above, we talk frequently about “ground-ball” and “fly-ball” pitchers. Grounders and flies are expressions of a pitcher’s control over launch angle. The relationship between a pitcher and his exit velocity is a bit more speculative, though.
Yesterday, I discussed how there was a detectable relationship between those two variables even looking at one year compared to the next. We also have a relationship (as discussed yesterday) between exit velocity and FIP, even if there’s also a decent bit of noise in there.
To see how the relationship with FIP works, it might be helpful to break down the components of FIP. The chart below depicts the correlation coefficient between average exit velocity and HR/9, BB/9, and K/9 for 186 single-seasons from 2015 and 2016 for the 93 pitchers who recorded more than 100 innings in both years.
Metric | r |
---|---|
K/9 | -0.19 |
BB/9 | 0.26 |
HR/9 | 0.39 |
While it’s possible that there’s some sort of relationship between strikeouts, walks, and exit velocity, that relationship doesn’t easily present itself in the data above. Where there does seem to be some sort of relationship is in home runs. Now let’s take a look at three groups from 2016: those with a high average exit velocity, those with a low average exit velocity, and a large group in the middle.
HR/9 | BB/9 | K/9 | |
---|---|---|---|
85.3 MPH-88.3 MPH (21) | 0.99 | 2.7 | 8.5 |
88.4 MPH-89.8 MPH (45) | 1.21 | 2.8 | 7.5 |
89.9 MPH-91.9 MPH (27) | 1.31 | 2.9 | 7.8 |
While the relationship between exit velocity and both strikeout and walk numbers appears to offer some promise, it might be better to consider them more deeply on another day. Not only is the coefficient lower for both those variables than for home runs, but strikeouts and walks exert less of an overall effect over FIP than homers.