PairWise Rankings explanation
The PairWise Rankings (PWR) are a statistical tool designed to approximate the process by which the NCAA selection committee decides which teams get at-large bids to the 16-team NCAA tournament. Although the NCAA selection committee does not use the PWR as presented by USCHO.com, the PWR has been accurate in predicting which teams will make the tournament field.
For Division I men, each team with a Ratings Percentage Index (RPI) of .500 or above is called a “team under consideration,” or TUC. For Division I women, the top 12 teams ranked by RPI are TUCs. The PWR method compares every TUC with every other such team, with the winner of each “comparison” earning one PWR point. After all possible comparisons are made, the points are totaled up and rankings listed accordingly.
The greatest number of PWR points any one team could earn would be one less than the number of TUC. Meanwhile, a team which lost all of its comparisons would have no PWR points.
Teams are then ranked by PWR point total, with ties broken by the teams’ RPI ratings. This tiebreaking procedure is in displaying the PWR but is not definitively the way the selection committee breaks ties. There is anecdotal evidence that the committee in fact uses RPI to break ties.
When it comes to comparing teams, the PWR uses four criteria which are combined to make a comparison: RPI, record against TUCs, record against common opponents and head-to-head competition. Starting in 2006-07, record against TUCs was included only if both teams in the comparison have played at least 10 such games.
For an example, let’s consider a hypothetical comparison between two teams, Alpha and Bravo:
Alpha vs Bravo RPI 0.5891 0 0.5933 1 TUC 8- 4- 1 0.654 0 8- 1- 1 0.850 1 h2H 0 2 COP 4- 3- 0 2.250 1 10- 3- 1 2.125 0 ==================================================== PTS 1 4
Bravo has the higher RPI and the better winning percentage against other TUCs (8-1-1, .850 vs. Alpha’s 8-4-1, .654). Against common opponents — teams both schools have played this season — Bravo actually has the better overall record (a .750 winning percentage, to Alpha’s .571) but Alpha gets the edge because it has the better individual winning percentage against common opponents. Starting in the 2011-12 season, the common opponent calculation compares the sum of the winning percentages against each opponent, as in this example:
Alpha (4-3) 2.250 --------------------------- vs. Charlie 2-0-0 1.000 vs. Delta 1-3-0 0.250 vs. Echo 1-0-0 1.000 Bravo (10-3-1) 2.125 --------------------------- vs. Charlie 3-1-0 0.750 vs. Delta 1-2-1 0.375 vs. Echo 6-0-0 1.000
Owning the RPI, TUC and common opponent categories each is worth one point, so Bravo leads Alpha 2-1 in these three criteria. (Note: Ties within each criteria are not broken.) Lastly, we add head-to-head competition, for which one comparison point is awarded for each win. Since Alpha and Bravo have played two games this year and Bravo has won both of them, Bravo gets two more comparison points and Alpha gets none.
Thus, Bravo wins this comparison by the score of 4-1, and gets one PWR point. Notice that the final score of the comparison itself doesn’t matter — Bravo gets only one PWR point no matter what the score of the comparison itself is. If the overall comparison were tied, the team with the better RPI would receive the PWR point.
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