* = 0.05 level of significance of 1997-98 difference. Any apparent inconsistency between the 1997-98 change estimate and the respective prevalence estimates is due to rounding error.

¹ For 12th graders only: Data based on five of six questionnaire forms; N is five-sixths of N indicated.

² Unadjusted for underreporting of amyl and butyl nitrites.

³ Unadjusted for underreporting of PCP (phencyclidine).

⁴ For all grades: In 1993, the question text was changed slightly in one-half of the forms to indicate that a "drink" meant "more than a few sips." In 1993, N is one-half of N indicated for all groups. Data after 1993 were based on all forms for all grades.

Source: Monitoring the Future study, Institute for Social Research, University of Michigan (December 1999).