Elia Powers:

Housing officers divided students into living groups, based upon major, age and other factors. So students who saw their list of 25, 50 or 75 (depending on the group) potential roommates already knew that those included would likely live on the same floor or at least in the same residence hall. It was their job — given names, e-mail addresses and personal Web pages — to pair off in a matter of three weeks.
When two students agreed on a match, they notified the housing office, which Cumia said most likely accommodated the request. (Inside Higher Ed)