Prior to 1991, the NHSDA did not sample from group quarters, such as college dormitories. College students were sampled in these earlier surveys (see Table 1), and dorm-dwellers had a chance of being sampled through their permanent addresses, but coverage of this population improved after 1990. We undertook trend analyses separately in the pre-1991 and 1991–2006 survey segments; no significant differences in trends among college students between these two survey periods were observed.
