Major-conference teams get a bonus from NET compared to RPI. Since the Committee has traditionally preferred teams from the majors, will those teams now get a “double bonus?”
As most anyone who follows college basketball knows by now, beginning in 2019 the NCAA has chosen to use its own in-house method, called “NET,’ to determine the seedings for the NCAA Men’s College Basketball Tournament. In past years the NCAA has relied primarily on “RPI,” along with conference memberships and major championships, to determine seeding.
RPI relies on a team’s won-loss performance, the performances of the teams it played, and the performances of those teams against their other opponents. It also has a simple formula which weights those three factors by 25%, 50%, and 25% respectively.
The NCAA has described the process by which NET ratings are determined, but they have not revealed the weightings or formulas used. Worse, the NCAA does not release each team’s actual score on NET. They only report the ranking of the teams on this concealed measure. This makes comparisons with RPI, which takes on specific numerical values, more difficult.
Rankings obscure the “distance” between adjacent teams. The number one ranked team might be considerably better than the second-place team, but that information is lost in rankings. For instance, here are the top-ten teams in 2018 by RPI and their rankings on that measure:
The “distance” as measured by RPI between the one and two seeds, and the two and three seeds, is essentially equal, at 0.01 on the RPI scale. That consistency soon breaks down once we look at teams with seedings or four or greater. Kansas and Duke were separated by only .0005 of an RPI point, twenty times less than the difference between Virginia and Villanova. Eighth-ranked Tennessee is fully 0.0066 behind Cinncinati, but only 0.001 ahead of Purdue. The NCAA’s decision to only release rankings on NET, and not numerical team scores, obscures these real differences in team performances.
That decision has forced me to treat the NET ranks as an “interval-level” performance scale like RPI, ignoring the problem of unequal distances just discussed. Here is the relationship between NET and RPI for the top-100 teams as ranked by NET.
The red line and red dots represent teams from the six major conferences, the Atlantic Coast, Big East, Big Ten, Big Twelve, Southeastern, and Pacific Twelve. The blue line and dots represent teams from other conferences. The labelled teams statistically deviate from their predicted NET ratings by 30 ranks or more.
Kansas is the obvious poster child for differences between the two measures. Kansas has sat atop the RPI rankings all season despite its 25-8 record because of its strength of schedule. Other teams like North Carolina State and Wofford have been substantially advantaged using NET when compared to their RPI ratings, while teams like Yale and St. John’s are significantly underrated.
The most obvious and important aspect of NET is its built-in preference for teams from major conferences. Major-conference teams, indicated by the red line, get NET scores on average ten full ranks better than teams from other conferences (the blue line) with identical RPI scores.
As the chart at the top of this article shows, the NCAA Selection Committee has consistently granted major and mid-major teams a substantial seeding bonus compared to teams from other conferences. Since NET already includes a bonus for major-conference teams, should we expect major conference teams will now get a “double bonus?”