According to the NCAA RPI, the Ivy League now is the #3 ranked conference, based on games played through October 17. And, after adding simulated results for all teams for games not yet played (including conference tournaments), it appears likely the League will end the season as the NCAA RPI's #2 ranked conference. On the other hand, according to the Massey ratings, the League currently is the #7 ranked conference. And according to my Balanced RPI ratings, the League currently is the #5 ranked conference and still will be #5 at the end of the season. So, why does the NCAA RPI rank the League so highly?
For a starting point, here are the non-conference data for the Ivy League teams. Below, I will comment on the data:
Its best win is against #154. Its best tie is against #18; and it also has a tie against #65. Its loss is against #44. There is nothing in its non-conference record that is consistent with its current NCAA RPI rank of #7.
As another example, looking at Princeton:
It has a win against #15, but its best win after that is against #100. It has ties against #42 and #161. It has losses to #2 and #71. Although the win against #15 relates well to its #13 rank, its other results seem inconsistent with that rank.
Going near the bottom of the League, in Cornell:
Its best win is against #194. It has ties with #147, #156, #167, and #206. None of this is consistent with its #136 RIP rank.
A close look at each team suggests that something is amiss with how the NCAA RPI is ranking the Ivy teams. Why is this happening?
Here is my explanation.
1. Non-Conference Winning Percentages. If you look at the School Non-Conference Record column, you will see that almost all of the Ivies have excellent non-conference records and all of them have positive non-conference records.
This is critically important. Remember, the RPI is based on three computations: a team's winning percentage (WP), its Opponents' Winning Percentages (OWP), and its Opponents' Opponents' Winning Percentages (OOWP). The WP has 50% of the effective weight in the RPI computation, the OWP has 40%, and the OOWP has 10%.
In conference play, Team A contributes its winning percentage to the OWP of each other conference team. And remember, the OWP has 40% of the effective weight in the RPI calculation. In addition, since the other conference teams also play each other, Team A's winning percentage also finds its way into the OOWPs of the other teams, although this has only 10% of the effective weight in the RPI calculation.
The point is, the very good non-conference records that the Ivy teams bring into conference play have a big impact on the each others' RPI ratings. And, the records of the teams against whom the Ivies achieved their good non-conference records are relatively unimportant since the OOWP effective weight is only 10%.
2. Proportion of Non-Conference Games. Once conference play begins, using a conference with a complete round robin as the best example, the OWP portion of teams' RPI ratings move towards 0.500. Consider Team A. It will play Team B and Team C. Teams B and C also will play each other. If Team B beats Team C, then Team B will add a win to its WP and Team C will add a loss to its WP. When Team A plays Team B, its OWP will gain a win; but when it plays Team C, its OWP will gain a loss. These also will find their way through to Team A's OOWP, although the overall RPI impact of that will be small. Because there always will be a matching OWP loss for each OWP win (or there will be matching ties), once conference play begins, the net impact of each game is to drive Team A's OWP (and OOWP) towards 0.500.
As a result of this phenomenon, the lower the portion of conference games that a conference has, the less drag there is of conference teams' OWPs (and OOWPs) towards 0.500. Since strong conferences' teams have OWPs above 0.500, this means that having a low proportion of conference games is good for strong conferences and having a high proportion is bad, from an RPI perspective. The following table, based on the years from 2013 (when the last major conference re-alignment was completed) through 2022, shows the proportion of conference and non-conference games for each conference (including conference tournament games):
As you can see, the Ivy League historically has had the highest proportion of non-conference games. Thus it has had the smallest proportion of games that are dragging its OWP towards 0.500. In other words, its relatively high proportion of non-conference games has given it an RPI advantage in relation to other strong conferences.
Conclusion. Because the OWP portion of the RPI accounts for 40% of its effective weight, with OOWP accounting for only 10%, when a conference's teams all have very good winning percentages, it inflates the teams' RPI ratings. And, since the OOWP effective weight is so low, it means it hardly matters against whom the conference's teams played in getting their very good winning percentages. Thus if all of a conference's teams play relatively soft non-conference schedules and by doing that are able to assemble very good winning percentages, the teams will receive RPI ratings and ranks that seem a lot higher than they should be when you look at whom they actually played.
And, this is compounded if the conference plays a high proportion of non-conference games as compared to conference games.
The Ivy teams' RPI rankings this year are a good illustration of these factors at work.
4-0 in the first round of the NCAA tournament. Looks like the Ivy teams' high RPI rankings are a good illustration of these teams being very good.
ReplyDeleteNot if you are meaning that the RPI rankings illustrate how good the Ivy teams are.
DeleteThe Balanced RPI, which ranks the teams significantly lower than the RPI, had all the teams winning their first round games. The Princeton and Columbia wins were based on their having home field advantage, which put their ratings better than their opponents after adjusting for HFA.
The RPI ranks the Ivy League as the #2 conference, whereas the Balanced RPI ranks it as the #5 conference.
Both the RPI and the Balanced RPI agree that the four teams should have been in the Tournament. But the Balanced RPI ranks the teams significantly poorer than the RPI: Harvard 19 rather than 11, Brown 20 rather than 8, Princeton 33 rather than 17, and Columbia 36 rather than 20.
There is no way to say from these four games alone whether the RPI or Balanced RPI rankings are closer to correct. The sample size is way too small. It is a good lesson on being careful about drawing conclusions -- or what conclusions to draw -- from a few games.
I should add an additional comment.
DeleteThe question I was responding to was about how good the Ivy League teams are. The purpose of the RPI, however, is not to tell how good teams are. Rather, it is to tell what they have demonstrated by their performance over the course of the regular season. The same is true of the Balanced RPI. That actually is a significant distinction.
My critique of the RPI, so far as the Ivy League is concerned, is that the RPI does a poor job of showing how well the Ivy teams performed over the course of the regular season, over-stating what they had demonstrated through their performance.