During the season, the Oakland Athletics give their pitching prospects what they call green sheets -- documents with different sets of statistics measuring how well the hurler is meeting objectives. Once a month, A's prospects are shown their first-pitch strike percentage, 1-1 conversion rate, walk rate, popup rate and ground-ball rate on this sheet.
As of now, FIP (Fielding Independent Pitching) is not on those green sheets. Based on the season No. 2 prospectA.J. Puk had -- especially when compared to traditional statistics -- the A's may want to consider adding it.

FIP was devised to strip defenders completely out of the equation when judging pitching performance. A squeaker through the infield can score a run and hurt a pitcher's ERA, even if the batted ball was out of the pitcher's control. So why not look at what's explicitly within the pitcher's control -- namely strikeouts, walks, hit-by-pitches and home runs?
That's how you end with the formula for FIP = ((13HR)+(3(BB+HBP))-(2*K))/IP + constant. Each component is weighted according to game impact. Home runs are more important events than walks, so they get a heavier weight. (FanGraphs goes deeper into FIP's equation here.) The constant used for the purposes of this story was 3.52, using data from the 16 domestic Minor Leagues in which top-100 prospects played in 2017.
These were the best five FIPs from top-100 prospects this season:

Pitchers among MLB.com's Top 100 Prospects (min. 50 IP)

Puk dominated in terms of FIP, telling a completely different tale than the more traditional ERA. The 22-year-old left-hander ranked 30th in ERA among the 34 top-100 pitchers who threw at least 50 innings in 2017, thanks in part to an unlucky .361 batting average on balls in play (BABIP) over 125 innings at Class A Advanced Stockton and Double-A Midland. 
Puk was magnificent, however, dealing with the aspects of the game within his control. The 6-foot-7 southpaw, who was taken sixth overall out of Florida in the 2016 Draft, struck out 34.5 percent of the batters he faced. That in itself will lead to a traditionally low FIP, but adding in the fact that he gave up only three homers over 125 frames (including 61 in the California League), a truly special FIP resulted. His 9.0 percent walk rate was pedestrian by comparison, but coming from a left-hander who had experienced control issues in college (10.9 career percent walk rate there), 2017 marked a step in the right direction.
In his first pro season, Puk couldn't have fit the A's mold for pitching prospects any better.
"People turn to me and ask, 'Gil, what's the best way to get someone out?' For most people, it's strike them out," A's Minor League pitching coordinator Gil Patterson said. "On the flip side, what's the worst way to put somebody on first? That's the walk. A bunch of pitchers can pound the strike zone and get a low walk rate, but that doesn't mean you'll get the strikeouts. That's OK. Give me a good ground-ball or popup rate, then. That's our philosophy with our pitchers."
The first thing Oakland got to working on with Puk as the organization prepared for his first full season was his mechanics.
The former Gators starter already had the stuff to miss pro bats. His fastball had been clocked in the upper 90s and his biting slider was already a plus pitch. The curve and changeup were more average, completing an arsenal that made Puk a solid starter.
To be more than solid, he needed to consistently find the strike zone. At 6-foot-7, Puk could use his size to overpower hitters but found it difficult to repeat his delivery. The A's believe they found a solution quickly.
"He would collapse his front knee so much, it was tough to repeat," Patterson said. "That front leg would bend 90 degrees or more, and by the end, we got it to barely bend and now he throws against it. That helps with a more consistent release point, but there's still some work to do on that. But I put together a before-and-after video the other day, and he'd completely changed."

"It's kind of how like David Price throws the ball," Puk said. "That was kind of the model we used."
The mechanical change remains a work in progress -- Puk's walk rate was 25th among the 34 top-100 prospects considered for this story -- but hitters had to honor the fact that the left-hander was working more in the zone. They still found him untouchable -- Puk's 38.6 percent strikeout rate in the California League and 30.8 percent mark in the Texas League topped both circuits among pitchers who worked at least 60 innings. 
Puk became even scarier after working with Midland pitching coach Don Schulze to add a two-seam fastball to his repertoire. The RockHounds starter credited the offering when he recorded 13 strikeouts in starts on Aug. 3 and Aug. 31. The pitch not only added a different movement to his potent fastball but had a similar look to an off-speed offering.
"I think I really hit my stride when I got comfortable with it," Puk said. "I think I throw my two-seam like my changeup. It's got the same look to it, and that can be really tough on hitters trying to square it up. Even when I got behind, I could throw my two-seam where I wanted for a strike, and that was huge."
The two-seamer wasn't just big for racking up K's, it also helped limit hard contact. Texas League batters hit .256 off the Midland lefty, but an incredibly high .380 BABIP played its part in that. Meanwhile, Puk saw his ground-ball rate jump from 40.5 percent at Class A Advanced to 47.6 percent at Double-A. His fly-ball rate, in turn, fell from 46 percent to 29.9 at the higher level. That drop helped Puk limit long balls -- he allowed only two homers in 64 innings with Midland, compared to one in 61 frames with Stockton.
"Even when I was in the Cal League, I was thinking about how the winds blow out and that I didn't want to make a mistake, so I didn't throw the two-seam at all because I didn't want to test it yet," Puk said. "Once I got to Midland, it became my main focus to get that going again."

Biggest ERA, FIP Differences Among top-100 prospects

Neither Puk nor Patterson agonized over the starter's ERA, which rose to 4.36 with the RockHounds, or the hits that continued to squeak through.
"That's kinda how baseball is," Puk said. "Sometimes, guys are going to crush the ball right off the barrel, but sometimes they're going to get it right off the cap and in for a single. That's out of your control. You have to know hits are going to happen. I'd obviously love to have a no-hitter every time, but as long as I stay within myself and don't get too frustrated, I'm OK."
"I blame it all on the shifting. Guys weren't in the right spots," Patterson said with a laugh. "But really as long as the damage isn't big, I'm not overly worried. He was getting his fair share of ground balls and popups, so that's what I'm looking for. Of course, I'd rather have the ERA somewhere around 2.00 or 3.00, but when you know there's little damage and soft contact, we'll be happy with where he's headed."
To Patterson's point, even when Puk gave up hits with Midland, his opponents were hardly crushing the ball. Of the hits he allowed at Double-A, 23.4 percent went for extra bases, well below the 2017 Minor League average of 30.5 percent.
For all those reasons, Puk will enter the 2018 season as one of baseball's most promising left-handed pitching prospects. For now, he was slated to participate in the A's instructional league but was sidelined for precautionary reasons by a forearm issue. He's expected to undergo a normal offseason throwing program and return healthy and ready for Spring Training. All involved are excited to see if he can post an ERA to match his skill set in 2018.
"When they fall behind, pitchers have a tendency to start thinking, 'Oh, shoot, here we go,'" Patterson said. "But with A.J., you can see him thinking, 'No, here I go.'"