It's that time of the year again -- the start of March Madness, 2018. Last Sunday, the selection show announced the 68 teams, and the preliminary round games start tonight. Of the 68 schools, 32 were automatically selected as conference champions, while the remaining 36 received at large bids.
What determines what 36 teams are chosen for at large slots? That generally boils down to won-loss performance against strength of schedule. Teams playing well against tough competition are rated highly. On the other hand, a solid record against mediocre competition might not make the cut. The 32 D1 conferences are quite unequal in hoops. The Atlantic Coast Conference (ACC) historically has been the cream of the crop, so one would expect multiple at large bids there. The Colonial Conference, on the other hand, might get no at larges, sending only the conference champ.
A month or so into the season, once teams have played ten games or so, the ratings systems such as the rating percentage index (rpi) kick in, providing a metric to rank all teams from 1 to 351 for basketball. rpi is composed of a team's winning percentage (25%), its opponents' winning percentage (50%), and the winning percentage of those opponents' opponents (25%). There's also an adjustment for home, away, and neutral venue games. A team's strength of schedule is thus critical to it's rpi potential, giving large school conferences a big advantage. Other metrics such as kenpom refine the rpi.
Since the 32 conferences are very unequal in their rpi and kenpom metrics, one would predict the "majors" to differentiate on rankings derived from rpi/kenpom, and subsequently to get more at large bids. And indeed that played out in spades this year, with the ACC hoarding 9 tournament slots (champion + 8 at large), the Southeastern Conference (SEC) 8, the Big 12 (B12), 7, and the BIG East (BE), 6.
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