When <= isn't an ordering: Example: teams rated on Offense, Defense, Stamina. Sets w/ no natural order: The ec-hw had some exercises from the book about partial orderings Eg, you can some things are bigger than others (we can say that Gwen Stefani >= Backstreet Boys, but we can't compare everything: Gwen Stefani !>= Leonard Bernstein, and Leonard Bernstein !>= Stefani. However, we do know that Stefani >= Backstreet Boys and Backstreet Boys >= N*Sync is enough to conlude Stefani >= N*Sync. That is, "better music than" is not a complete relation, but it is transitive. However, sometimes you have a relation that seems like it should be transitive, but it's not! For example: suppose Ultimate Frisbee teams really have three underlying ratings: Offense, Defense, and Stamina. And suppose that when two teams play, whoever has a better rating in two out of the three categories is the winner It seems like the relation "beats(X,Y)" should always be a nice, complete ordered relation (especially since we've eliminated all randomness and psychology present in real-world sports), but it's not!: Say team A has offense=3, defense=2, stamina=1. team B has offense=2, defense=1, stamina=3. team C has offense=1, defense=3, stamina=2. We can see that A > B, and B > C, yet C > A. The upshot: be careful about trying to project multidimensional data down to a single scalar; you'll induce an ordering which may not reflect the original multi-dimensional relation. (And the original relation might not qualify as an ordering -- not even a partial ordering -- even though it sure seems like it should be one.) Alternate upshot, to console Rockets fans: It's conceivable that every team in the league is equally good, and it's merely the order of the matches which is determining the outcome. ======================= Snippet from Richard Tapia's University Professor talk, 2005.Nov.29: ***"Rankings are evil."*** Humans are many-dimensional; you can't meaningfully reduce them to a single number (or even, just their academic talent/creativity/ability). To try to say that every student with a 1425 SAT should be accepted before students with 1400 SAT is ludicrous — and, ludicrous in a way which is socially damaging. Instead, set a threshhold or bin (say, 1350-1450), and then view everybody within that bin as potentially equal-scoring on the test. (Similarly, w/ faculty hiring: schools which hire only the candidate from the top-ranked grad schools are missing out on a lot of diversity and talent from students coming from schools ranked #6. So set a threshhold of competency, then choose from that entire pool.)