Last year the NCAA introduced a new method of rating NCAA mens’ college basketball teams called “NET” and supposedly stopped using RPI as its principal criterion for seeding the annual Tournament in March. In other posts here I’ve shown that RPI was the dominant influence on seeding before 2019 with adjustments made depending on the conference to which a team belonged and whether it won its conference championship.
The Significance of NET
The NCAA’s adoption of NET made it the most powerful predictor of Tournament seedings in 2019, but both a team’s RPI and whether it had won a major conference championship continued to influence the seedings as well. For a few prominent teams the gap between the two measures was enormous. RPI adjusts heavily for a team’s “strength of schedule” while NET uses a variety of criteria.
Last year, Kansas entered the Tournament with the nation’s second-best RPI but ranked only twentieth in NET, no doubt influenced by the team’s underwhelming 25-9 record. Its high RPI reflected its schedule, the toughest in the nation last year. In past Tournaments a team with the second-best RPI would usually be seeded on the top line. Now, with NET the dominant factor, Kansas received only a four seed and won just one game.
Divergent cases like Kansas might prove informative. I have compared the NET and RPI ratings from teams participating in the 2019 Tournament, the only one to use NET so far. I excluded conference champions who receive automatic bids to the Tournament and used only the remaining 34 teams that received an “at-large” bid. As always, I excluded the four play-in games for two eleven and two sixteen seeds.
When NET and RPI diverge
There are two methods to measure the divergence between two rankings, their difference and their ratio. I’ve chosen to use the ratio of the NET ranking to the RPI ranking rather than the difference between them. A difference of two ranks probably means more at one and three rather than nine and eleven. Using the ratio accounts for that intuition. The ranks one and three have a ratio of 0.33, very low as we’ll see shortly. A pair of teams at nine and eleven score a more mediocre 0.82. On this measure, lower values are better since the score is based on rankings, just the same way being ranked first is better than being ranked twentieth.
I have grouped the 34 teams into three categories. The first row presents results for teams whose NET ranking was considerably worse (larger) than their RPI. Kansas is the poster child for this measure; it has a ratio of 20 because it was rated twentieth on NET but first on RPI (=20/1). On the bottom row we have the results for teams who were viewed much more favorably on NET than RPI. Virginia Tech, Gonzaga, last year’s runner-up Texas Tech, and last year’s champion Virginia all belonged to this favored group.
Because the samples are small, and the results limited to just one year, I make no broad claims about the applicability of these findings. Still, teams with better rankings on NET than on RPI appear to have won more games in last year’s Tournament. Teams whose NET rankings were considerably larger than their RPI rankings won an average of only 0.8 games in 2019. Teams whose NET rankings were considerably smaller than their rankings on RPI won an average of 2.0 games.
Some of this difference in games won reflects the seedings the teams in each group received. Teams with low NET/RPI ratios were seeded on average at 5.3 while those higher ratios averaged worse seedings of 5.8 and 7.0. Because seeding is the most powerful determinant of Tournament victories, better-seeded teams should on average record more wins.
To estimate the effect of seeding I predicted each team’s number of wins in 2019 using the base-two logarithm of its seed. For a top-ranked team the prediction is four wins, or entrance into the Final Four. Teams with seedings below seven are predicted to have a negative number of wins. I have truncated these to zero just as the Tobit procedure I use does when calculating the predicted number of wins in the table.
For teams in the two groups with NET/RPI ratios above 0.7, seeding alone predicts a nearly identical number of wins. But for the teams with lowest ratios, the prediction based on seeding is just 1.4 wins, well below the 2.0 wins we observe. That suggests that seeding alone cannot fully account for the success of these teams whose NET ratings were substantially better than their RPIs.*
NET and RPI in 2020
Auburn this year looks like Kansas from 2019. As of March 3rd, Auburn currently has the third-best RPI but ranks only 27th by NET. Unlike last year’s Kansas team, Auburn has a gaudy 24-4 record but only the 44th-ranked strength of schedule. Based on NET alone, we might expect Auburn to draw only a six or seven seed in this year’s Tournament. A seeding based on RPI alone would put it on the “two-line.” Auburn’s actual seeding will tell us a lot about how the Tournament Committee values NET and RPI rankings.
Here are the NET, RPI, and ratio values for the top-25 teams as ranked on NET. Gonzaga, Michigan State, and Texas Tech look like possible Tournament over-performers while Villanova, Florida State, and Oregon may disappoint their fans.