In a sport where empirical analysis has become vitally important, little is now taken for granted within baseball. Indeed, one of the central tenets of sabermetrics is to question everything, to challenge even the game’s longest-held assumptions in the hopes of correcting any unseen biases. Conventional wisdom isn’t so much attacked as it is scrutinized and re-examined in a manner where faulty generalizations are remedied and room is made for nuance.

In the world of scouting, the jump from High-A to Double-A has long been seen as the most critical for a prospect’s future. Double-A, the thinking goes, is where a minor leaguer’s true ability is tested for the first time against competition that can also list “future big leaguer” as a realistic goal.

It’s the level where weaknesses are often uncovered — that hole in a hitter’s swing now exposed, that pitcher’s inconsistent command now a problem that needs fixing. If you excel in Double-A, there’s a pretty good chance you’ll get a shot in the majors, and sometimes you’ll even bypass Triple-A altogether.

But why is Double-A the proving ground for prospects? Why is this level the place where the game’s most talented youngsters truly cement themselves as major-league ready?

Why, in other words, are the adjustments made at Double-A so telling and significant for a prospect? And is this still the case, as conventional wisdom has long assumed?

“Double-A isn’t a magical level where unexpected things happen,” FanGraphs’ lead prospect analyst Kiley McDaniel told me via email. “It’s more a product of how the minor league promotion system works.”

This reality becomes more clear when looking at the makeup of Triple-A squads, which aren’t stocked with prospects but rather used as “more of an inventory level for big league rosters,” as McDaniel puts it. While plenty of future major league stars make their way through Triple-A, the level is also filled with failed and fringy big leaguers who are in their late 20s and early 30s — aged long past the point where they can still be considered legitimate prospects. Every once in a while, said players will even become folk heroes.

A step below in Double-A, however, teams are still looking to shape future big leaguers from less fully formed balls of clay. Rarely do you see players aged into their late 20s (except on rehab assignments), and the overwhelming majority of players on the roster have a reasonable chance of reaching the majors.

As a result, McDaniel says, “Double-A is the highest level where development is still happening.”

Further down the minor league ladder in A-ball, the competition isn’t nearly so consistent and challenging, especially for baseball’s most talented youngsters.

Speaking about the differences between the lower levels and Double-A, long-time prospect writer John Sickels talked about the “winnowing process” that has occurred once players move into the upper minors.

“This sounds banal, but it is true: The players are simply better [in Double-A]. At the A-ball level there are many ‘organization players’ and roster-filler types who aren’t likely to succeed at higher levels…pitchers with 86 mph fastballs, position players who can defend competently but don’t have impressive bats, etc.”

Sickels also pointed out how there are 60 rosters at the A-ball level, but just 30 Double-A teams. “There are half as many roster spots to fill. It is the key step in distinguishing roster-fillers from genuine prospects.”

So what differentiates a minor leaguer who performs well at Double-A from those who simply succeed against weaker competition down in the lower levels?

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For one, hitters have to show they can excel against pitchers with plus velocity and good secondary offerings.

“A Double-A pitching staff will have more genuine prospects, more guys throwing in the 90s, more guys with refined breaking pitches and more guys with sharper command,” Sickels said.

Consequently, young hitters who feast on easier competition due to athletic ability alone often have difficulty making adjustments.

As Al Skorupa, a member of Baseball Prospectus’ prospect team told me, “In A-Ball, some guys have loud enough tools that they succeed without refining their approach.” He said that’s no longer the case in Double-A, where pitchers “really throw more strikes and locate a lot more consistently.”

Given that pitchers possess better command across the board, hitters aren’t able to succeed by taking tons of pitches and getting ahead in the count against hurlers who don’t consistently locate their fastballs.

“Lots of hitters will show a falsely good approach in the low minors,” Skorupa said. “Those guys get promoted and suddenly you’re not waiting out Double-A pitchers like that. Suddenly you’re down 0-2 a lot and your slash line plummets. Most of these issues aren’t exclusive to the Double-A jump, of course. All these issues exist at every level jump to some degree. It’s just at that level, we see the biggest degree of differences.”

Of course, pitchers face similar challenges upon reaching Double-A. Where once they could dominate lower-minors competition solely with big velocity, hurlers now have to mix in secondary offerings and sharpen their command.

“One noticeable change for pitchers moving from High-A to Double-A is the ability to consistently locate a breaking ball while also throwing around 90 mph,” McDaniel said. “Those two abilities alone will likely get a pitcher through the A-Ball levels pretty easily.”

Just as hitters face a tougher brand of competition, so too do pitchers have to prove that they can continually succeed against better quality opposition. At a level where batters are beginning to develop a smarter, more refined approach, young hurlers can’t overmatch batters without their own well-executed plan.

“A typical Double-A hitter is less likely than his A-ball counterpart to chase junk pitches outside the strike zone,” Sickels said. “You’re more likely to find hitters capable of handling major league quality fastballs; there are fewer weak bats that you can just overpower. If your command isn’t sharp, or if you don’t have something to go with your fastball, those weaknesses will get exposed quickly in Double-A.”

As a result, fastball command and the consistent ability to get ahead in the strike zone grow in significance once a pitcher reaches the upper minors. The emphasis is less on developing and improving upon new offerings and turns instead toward sharpening up the rough edges remaining in one’s game.

Development, in other words, is no longer always the primary motivation. With the bright lights of the majors beckoning, the sharpening of tools into usable big league skills takes a more central role. And this notion — that Double-A is the place where the focus begins to shift toward the major leagues — is what separates the upper minors from A-ball. Adjustments are now made with a view toward how one can contribute at the major league level. Room for growth still remains, especially among the youngest players, but Double-A is where prospects begin preparing for how they’ll contribute in the majors.

As Skorupa told me, “In Double-A, it’s time to stop focusing on developmental issues with players and shift focus to ‘how can this player help the major league team in the near future?’”

“You really are just a phone call from the majors once you hit Double-A, so we’ve gotten to the point there where teams are thinking of your place on the depth chart.”

With the game far more competitive between the lines, players also have to take their preparation off the field more seriously. When a prospect reaches Double-A, there are few guys still remaining who can be labeled as “projects.” Players who have the talent but not necessarily the work ethic and/or desire to improve on their weaknesses rarely reach the upper minors.

“Players really need to have their heads on straight by Double-A. You can’t be someone who’s not taking his prep work seriously at this point,” Skorupa said.

Indeed, the increased professionalization of the game at Double-A is another challenge prospects face. The talent level is more uniform in a way many minor leaguers have never encountered before, with few youngsters able to succeed on their athletic ability alone anymore.

For the first time, a prospect who has long dominated his peers, from Little League to high school to the lower minors, might have to experience extended failure. A player’s aptitude for making adjustments, putting in the extra work and being open to advice from coaches only grows in importance.

What this underscores is the tremendous nuance in player development and all the adjustments prospects have to make — both on the field and off — at a young age to succeed against increasingly tougher competition. Just about every player who reaches Double-A has the physical skills to play in the big leagues, but those skills become less and less of a separating factor the further one climbs up the minor league ladder.

This is what makes projecting a prospect’s future performance so difficult, of course. The process of determining why one prospect failed while another thrived, why one youngster makes the necessary adjustments and the other can’t is remarkably opaque. So many factors affect a young ballplayer’s maturation, and organizations are just starting to focus on those.

Since hiring Gabe Kapler as the club’s director of player development last offseason, the Dodgers have begun concentrating on numerous new ways to aid the growth of their prospects. They memorably posted a sign in their dining room during spring training that declared the club as “the healthiest team in pro sports,” with Kapler emphasizing the importance of nutrition and eating healthy. Under Kapler, the team is also stressing the value of communication and mental health, two factors that have been frequently overlooked in player development.

You can bet the Dodgers aren’t the only organization bringing progressive thinking to the minor leagues, even if the club’s efforts are the most publicized. The impact these different approaches ultimately have on prospect development remains to be seen.

This season, of course, a bevy of talented youngsters have debuted in the majors, and many are making a big impact from day one. Plenty of rookies, moreover, have gotten their initial exposure to big league competition straight from Double-A in 2015, with the likes of Miguel Sano, Byron Buxton, Michael Conforto and Kyle Schwarber all skipping over Triple-A to contribute to their big league teams.

From this perspective, the significance of Double-A on a prospect’s path to the majors hasn’t changed one bit. If anything, the upper minors have grown only more competitive and more vital to a youngster’s development into a major league-ready player. As Jeff Zimmerman has shown at FanGraphs, players are now hitting their primes earlier, with their best performances coming at younger ages than we’ve traditionally seen.

Much of this is the result of how players are developing down on the farm, and Double-A is still the place where a prospect must make the most crucial adjustments to his game. Not every minor leaguer who excels at Double-A will turn into a major league star, but the level still tells us the most about a prospect’s outlook.

With baseball’s best players only growing younger, Double-A remains the proving ground where prospects are shaped into major leaguers.