Recently at ESPN.com I offered some forecasting on what shape college basketball may take next year when the term “power five” is, at last, accurate in basketball. In summary, the Big 12 might continue to swagger smugly around the top of the KenPom leader board, the ACC may again bring up the rear, and in between these poles the already fast-improving SEC could be strengthened further still by virtue of expansion alone.

Whenever pieces concerning the major conferences are posted, it is customary to field responses saying this or that additional league should also be regarded as meriting the label in question. Fair enough. Here is one working definition:

Over a five-year period the mean of a major conference’s performance will be equivalent to outscoring an average Division I opponent by at least 10 points over 100 possessions.

Scoring margin over 100 possessions is of course a nod to what’s shown as AdjEM (adjusted efficiency margin) at KenPom. Tracking this over five-year windows prevents one-season outliers from wagging the dog, and for that the ACC is thankful. Otherwise the unsightly +8.58 the league coughed up last year would have resulted in the ACC’s major-conference membership card being revoked.

One salutary aspect of this definition is that, whatever its descriptive merits, it at least draws a clear line. Indeed, our innate human predilection for setting boundary lines at round numbers divisible by five has rarely offered such rich dividends in the area of cartographic tidiness.

Five-year average adjusted efficiency margins, 2003-24

                 Five-year averages    Five-year averages
                   +10.00 or above        below +10.00
Big six leagues          131                   1  
Rest of Division I        0                   560

Data: KenPom

The Pac-12’s five-year average dipped to +9.97 in 2019. Other than that our usual six suspects have a perfect record of clearing this threshold every time over the last two-plus decades. Conversely the best number any other league has shown since 2003 is when the Mountain West’s five-year average peaked at +9.13 in 2013.

Then again patterns that that have held true for a while are hardly impervious to change. For the past two seasons the aforementioned Mountain West, for example, has been performing at a statistical level (between +9.00 and +10.00) more or less equivalent to that of a down-year major conference.

We’ve seen runs like this from ascendant leagues before. Thus far, these runs have not been sustained. The Mountain West itself posted a remarkable +11.79 in 2013 only to be reduced to one-bid-league status within three years. The six-bid Atlantic 10 of 2014 claimed head coaches like Dan Hurley and Shaka Smart and bore a strong resemblance to today’s Mountain West. But, again, the league in question was unable to maintain that level of performance.

Possibly the current Mountain West will break this rise-and-fall tendency even in the face of a definition of “major conference” that’s purely relational, hinging as it does on how well you play collectively against the rest of Division I. There will never come a day when a surpassingly large fraction of accredited men’s basketball programs outscore D-I’s mean level of performance by 10 points over 100 possessions.

Still, it’s no less true that both D-I and the sport itself globally are growing. It may be the case in the present tense there’s room for conferences with a total membership of 90-something teams to attain the honorific of “major.” And if this is not the case now, it soon could be.

After all, give old people in 2024 any opening at all and they will bang on about Conference USA in the early aughts. Back then the league was loaded with Cincinnati at its Huggins-era peak, Louisville, Marquette, Memphis, and DePaul when the Blue Demons mixed down years with decent NCAA tournament seeds. Other than the Tigers all of the above decamped to the Big East (itself freshly pillaged by an ACC harvesting more football), but for one shining moment you could squint and perhaps see Conference USA as still another major conference.

Even when the Mountain West or some other rising league picks up this fallen standard and joins D-I’s top tier, however, we will likely still be left with the same definition of mid-major. It’s a definition that one’s been able to use, if one wishes, with a fair degree of utility for a while now.

A mid-major is a team whose projected seed is significantly lower than its NCAA team sheet metrics would predict.

The reference to “metrics” here is offered strictly as agnostic reporting: NET, KPI, SOR, BPI, KenPom. These are the numbers the NCAA affirmatively puts in front of the committee. The men and women on the committee can of course roam wherever they wish online, but these are the numbers that bear the official imprimatur.

Plainy this definition of mid-majors is nowhere near as water-tight as the major-conference screening device hazarded above. Indeed it’s not even equally adept in both the “pass” and “fail” functions. Each year a tiny number of “true” mid-majors do achieve escape velocity and nab a seed in line with the story the NCAA’s metrics tell about that team’s performance. The definition offered here, in other words, will definitely miss the occasional mid-major.

But when our handy mid-major detector does beep with a positive reading, it’s pretty trustworthy. Go down a projected (or, eventually, actual) NCAA tournament bracket from the very top line to the longest of long-shots on the far edge of the bubble. Stop at every instance where the metrics would predict a significantly higher seed than what you’re seeing projected. Often the explanation will be that the team in question is a mid-major.

One thing we’re saying when we label a program as “major-conference” is that we will believe your metrics on sight.