My interest in the mathematics of sports ratings began with Boyd Nation's article on the Distance Matrix. Since I read that some years after it was published, I have used metrics based upon the "incidence matrix" to analyze the data available to computer rating systems.
Kenneth Massey shares my concern (or more appropriately I share his) that the trend toward fewer interconference games and more FCS non-conference opponents makes it more difficult to compare FBS teams that haven't played each other. Dr. Massey made available to me gamescores for all "big college" games from 1960 through 2011. I used Dr. Massey's historical gamescore data to calculate two of what I believe to be important measures of the games graph. %O+OO is the percentage of the field that is "connected" by no worse than an Opponent's Opponent relationship (higher is better - ideally this should be greater than 50%) and APL is Average Path Length between any two teams (lower is better, ideally it would be 2.)
It is not hard to explain why the field was so weakly connected in the 1960s and 1970s:
For the 2006 season and beyond 12-game schedules were instituted on a permanent basis (we'd like to think because they noticed what we analysts did, but we analysts suspect it was more for the extra money.)
So why did the connectivity metric only rebound to the 1990s level? Or for that matter why for the intervening two 11-game years did it drop back to the 10-game 1980s level? Before I answer that question, it is profitable to look at not just how many steps along the path it takes to connect the field, but how many different paths by which each team-pair is connected.
Simple. At about the same time, the rules were changed to allow a win over a non-FBS opponent to count towards bowl-eligibility every year instead of once every rolling four-year period. In 2002, about a quarter of FBS teams had an FCS opponent, in 2011 over two thirds did.
A 12-game season is about the right number of games to connect a field that includes ~120 teams, but not if most teams don't play 12 games against the field.