Here is the relevant passage in the link that I posted. Please let me know if you need a little more plain English explanation.
As the table shows, the spreads for the three elements grow smaller when progressing from Element 1 to Element 3. The reason for the diminishing spreads is obvious, if one thinks about it. The computation of Element 1 looks at one team's record. Individual teams' records reasonably can range from undefeated (an RPI Element 1 of 1.0000) to all losses (an RPI Element 1 of 0.0000), for a maximum reasonable (though not average) spread of 1.0000. For Element 2, the computation looks at about 19 teams' records and averages them out. Teams, on average, play about 19 games in a season. With this many teams' records being used for Element 2, nearly all of the teams are going to have some wins and some losses, so the high Element 2 is going to be less than 1.0000 and the low is going to be higher than 0.0000. Similarly, for Element 3 the computation looks at about 361 (19 x 19) teams' records. This inclusion of a very large number of teams' records produces Element 3 numbers that are even less at the extremes than for Element 2, making Element 3's maximum reasonable (and average) spread smaller than for Element 2 and much smaller than for Element 1.
At the bottom right of the table, the yellow highlighted numbers show the average effective weights of the three elements over the 11 year period covered by the table, when the three elements are incorporated into the RPI formula using the 25%-50%-25% formula ratios:
Element 1: 49.6% -- roughly 50%
Element 2: 38.9% -- roughly 40%
Element 3: 11.5% -- roughly 10%
If you are having trouble understanding this, think of fruit salad. I want my fruit salad to consist of 50% cantaloupe, 40% oranges, and 10% kiwi fruit. To do that, I compare the fruit sizes and figure out that the right ratio of ingredients is 1 cantaloupe to 2 oranges to 1 kiwi fruit. In this analogy, 1 canteloupe = 1 x RPI Element 1; 2 oranges = 2 x RPI Element 2; and 1 kiwi fruit = 1 x RPI Element 3.
The 50-40-10 percentages suggest that the NCAA adopted the 1:2:1 weights in the formula for the three Elements in order to have a team's winning percentage count for approximately half the team's RPI (Element 1's roughly 50% effective impact) and the team's strength of schedule count for the other half of the team's RPI (Element 2's roughly 40% effective impact plus Element 3's roughly 10% effective impact). In a January 23, 2009 Memorandum from the NCAA's Associate Director of Statistics to the Division I Men's Basketball Committee, the NCAA confirmed that this is its intention: "About half of the rating is based on winning percentage and the other half on strength of schedule. Winning percentage (Factor I) only receives a 25 percent weighting although its real strength is larger. There always is a far wider gap in the rankings between the top and bottom teams in this category than between the first and last in Factors II and III."
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