Last Tuesday at Wrigley, Jake Arrieta fanned 13 Reds and faced only one batter over the minimum in a masterful one-hit shutout. That game crowns an impressive campaign in which Arrieta has posted this line, with every one of his rate stats a career best.

More on Arrieta’s breakout year after the jump.

That line is a far cry from Arrieta’s work before this season, which looked like this.

So, is it unusual for someone with a career like Arrieta’s to put it all together with a season like this one? To answer that question, I first considered how unusual this season has been. It didn’t take long to realize it’s not one you see every year (or even every decade). Say what? Well, ask yourself  how many Cubs’ starters have a live ball era 150+ IP season with WHIP under 1.1 and HR/9 under 0.4. In fact, there’s only one, and it’s someone you might have heard of.

So, pretty unusual, particularly for a pitcher whose prior performance hadn’t exactly set the world on fire. In fact, with an 80 ERA+ in 400+ IP thru age 27, Arrieta’s career was in definite jeopardy. Lest you doubt me, consider this list:

Those are the starting pitchers since 1920 to finish their age 27 season with 400-600 IP and an ERA+ of 80 or lower. Those are also the career totals for Hartung and Pole, and almost the career totals for Rupe (he got 10 more IP). Morton survived an age 28 season of 81 ERA+ to become a regular in the Pirate rotation the past two seasons.

But, that is admittedly a pretty small group. To develop a proper comparison group, I adapted those criteria and compiled a list of pitchers who, at least once in their careers, finished a season with these markers:

• aged 29 or younger
• started at least 60% of career games
• career IP between 400 and 800
• career ERA+ below 90

In fact, there have been 176 such pitchers (including Arrieta) since 1920 (I included a few pitchers whose careers started before 1920, but who would still qualify if their pre-1920 service was included). You can see them here, showing the season when their career stats first qualified them for this group. Mostly, it’s a nondescript bunch, with the notable exception of two HOF members, Tom Glavine and Red Ruffing (who both just qualified, with an 89 ERA+), and a few other notables (Jamie Moyer, Jerry Reuss, Rick Wise). Arrieta’s 80 ERA+ after the 2013 season is tied for 22nd lowest mark in the group.

To show how modest this group’s accomplishments are, Arrieta will be only the twentieth of those 176 to record a season of 150+ IP and 125 ERA+ before the age of 30. Of the other 19, only Glavine had more than one such season. Relaxing the criteria to find such a season at any point in a career adds only twelve more names.

If we bump up the ERA+ threshold from 125 to the 140 level that Arrieta will most likely hit, only 11 pitchers in the group have such a season, and only four did so before the age of 30. Of those four, one is anomalous (Oliver Perez actually had his 140 season before qualifying for the group), one is Glavine (of course) and the others are Mike Moore and Ricky Bones. Only Bones matched Arrieta’s feat of recording that 140 season the year after first qualifying for this group.

Career value in the group is equally sparse, with only these eleven compiling 20 WAR for their careers.

So, perhaps cast a glance Arrieta’s way next year and see how he follows up this season. He might indeed be the rare late bloomer who is just starting to put it all together.