Perhaps the most unique aspect of minor league baseball is the wide array of offensive environments. The hitter-friendly California League stands in stark contrast with the Florida State League, where runs are scarce and pitchers generally thrive. Both leagues operate at the high Class A level, but the average Cal League team this year scored nearly a run more per nine innings (0.95) than their FSL counterpart. That works out to 133 runs over the course of a 140-game season.

To put that in further perspective, consider that the FSL’s most productive team, Dunedin, would have ranked dead last in the Cal League with their 610 runs. But if we bolster their attack by that 133-run exchange rate, the Blue Jays would surge to fourth in the Cal League with 743 runs. And that doesn’t even take into account that because of the FSL’s myriad rainouts, Dunedin completed six fewer games than High Desert, the Cal League’s most offensive outfit. Furthermore, that doesn’t take into account the number of seven-inning contests, brought about by doubleheaders, played by Dunedin. We do know that Mavericks pitchers completed more than 90 additional innings (92 2/3 to be exact) than Dunedin hurlers.

League context is crucial to the process of ranking prospects. So as you digest our various league Top 20 Prospects lists, you can consult the chart below to see how players compare with the league averages.

• R/9 scales runs scored to nine innings, making no distinction between earned and unearned runs.
• The BB and SO columns figure walks and strikeouts per plate appearance. Walks do not include intentional passes.
• Isolated power (ISO) is the difference between slugging and average, separating extra bases and weighing them per at-bat.
• Balls in play average (BIP) figures the rate at which struck baseballs—excluding home runs—evade defenses and are scored as hits.
• The HR and RUNS categories are league averages for team home runs and team runs scored, providing a snapshot of the various offensive contexts.

Looking at full-season leagues averages, we see a lot of trends that make intuitive sense. Strikeout rate plummets and power output increases as players move up the minor league ladder. Defensive efficiency, as measured by BIP, also improves slightly at higher levels. That is, balls put in play at the Triple-A level fall in for hits less frequently than they do in low Class A. (Because of their high-octane nature, the California and Pacific Coast leagues serve here as outliers that disrupt these trends.)

But the one league comparison that seems to defy explanation occurs at the low Class A level, where the Midwest League proved to be a better run-scoring environment than the South Atlantic virtually across the board. In most years, the opposite is true—and dramatically so. (Take 2007, for example.) The average age of batters and pitchers in these two circuits is practically identical, so it’s not a matter of experience. Whatever it is—changing weather patterns, uneven distribution of talent, new ballparks—it merits further analysis.

A player’s age relative to his level of competition goes hand-in-hand with league context in making these prospect evaluations. Courtesy of Baseball Reference’s wonderful minor league site, we present the average age (weighted by playing time) at the various levels of the minors.

That’s not a misprint. By virtue of the Arizona League having 11 teams, the Rookie complex level has an uneven number of teams.