Jake Arrieta Has Actually Been Really Good, Primed to Be Better

Watching the Cubs rotation give up bomb after bomb from the seats of Dodger stadium — yes, I was one of the 14 early-arriving fans — sent me into a frenzy. Jake Arrieta in particular, who has a 4.92 ERA with a much lower 4.00 FIP, can’t be this bad going forward. That’s what I kept telling myself.

Indeed, after diving into the data, I realized that my built-up anxiety over Arrieta’s recent starts has been misguided and overblown. I was convincing myself that the velocity drop — which is still a trend to monitor — is to blame for his struggles, but I was wrong. Actually, lower velocity hasn’t been a big problem. So what’s the real issue?

Arrieta’s high ERA is the product of two factors: 1) an unsustainable fly ball rate and 2) an unsustainable home run rate. Those might sound like the same thing on the surface, but let’s dig into the details a little more to see what they mean.

Unsustainable fly ball rate

The bearded Cubs pitcher put up grounder rates of 56.2 percent in 2015 and 52.6 percent in 2016. A combination of both high whiffs and turfed batted balls enabled the pitcher to have phenomenal ERAs in those two seasons (1.77  3.10, respectively). This year, perhaps surprisingly, Arrieta’s whiff rate (75.9) has been nearly identical to his Cy Young campaign (76.1).

The difference between the years is a ground ball rate that is 27 percent lower (from 56.2 to 41.1). Every one of Arrieta’s pitches is inducing fewer grounders, as illustrated by the graph below.

The decline in four-seam grounder rate looks like it took an escalator down a few floors, but I want you to pay less attention to that. Focus instead on the slider and sinker drops, as those two pitches make up 70 percent of Arrieta’s repertoire. With this in mind, we need to address two questions:

1) Are his slider and sinker being thrown with less movement?

2) Are those pitches elevated?

Answer to Question #1

The 2015 Cy Young award winner’s slider and sinker are not moving less. Sinkers are tailing just as much as in previous years and sliders are cutting in to lefties with similar action.

Not only is Arrieta’s horizontal movement similar, but the dropping action of both the slider and sinker has actually increased this year.

Answer to Question #2

Just as Arrieta’s sinker and slider have nearly identical movement, so too is the location relatively unchanged. This year, he’s mostly throwing sinkers below the belts of batters, and he’s diving sliders towards the dirt, similar to what he did in prior seasons.

What’s more, he’s not leaving pitches over the middle of the plate, as evidenced by the horizontal location graph below. Notice how linear the graph remains, meaning not much has changed between the seasons.

Unsustainable Homer Rate

Arrieta’s HR/FB rate is 16.11 percent after his start in Los Angeles, which is 45 percent higher than last year’s mark. The spike in homers is what has contributed to the domineering pitcher’s 4.01 FIP, but this is BS and shouldn’t continue going forward if everything else remains the same.

Of course, fans can point towards Arrieta’s xFIP of 3.57 and conclude that he’s been unlucky with the amount of fly balls that have left the yard. But xFIP is imperfect in that is assumes that each fly ball has a 10.5 percent chance of being a homer.

Now that we have scFIP (Statcast FIP), though, xFIP has become pretty obsolete. scFIP calculates the exact probability of each fly ball leaving the park based on location, exit velocity, and launch angle. Arrieta’s scFIP is a robust 3.45, and is clearly in sharp contrast than his actual 4.01 FIP number. What this suggests is that the same fly balls wouldn’t be homers in other ballparks or under different conditions.


We’ve established several truths here. First, Arrieta’s sinker and slider are still being thrown away from the middle of the plate and low in the zone. Second, both of these pitches are being thrown with the same movement as in the past. Third, his scFIP of 3.45 suggests that a 16.1 percent HR/FB rate is not an accurate reflection of his talent.

As the season progresses, there is a great chance that Arrieta’s grounders start to increase and his homer rate takes a sharp nosedive back to the mean. Arrieta said he feels like he is close to where he wants to be and if a 3.45 scFIP is not where he wants to be, then we could be primed for even better run prevention.

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