Study Design

The core feature of the design is an annual survey of each new high school senior class, beginning with the class of 1975. Each year approximately

16,000 seniors are surveyed in approximately 135 public and private high schools that have been scientifically elected to provide an accurate, representative cross section of high school seniors throughout the coterminous United States. Data are collected following standardized procedures via closed-ended questionnaires administered in classrooms by University of Michigan representatives and their assistants.

In 1991, the project was expanded to include nationally representative samples of students from the eighth and tenth grades as well as from the twelfth grade. Approximately 18,000 eighth graders and 16,000 tenth graders are surveyed annually, using procedures similar to those used in the twelfth grade surveys.

One limitation of the design is that it does not include in the target population the young men and women who drop out of high school before graduation, and who make up between 15 and 20 percent of each age group nationally, according to U.S. Census statistics. The omission of high school dropouts does introduce biases in the estimation of certain characteristics of the entire age group, but, because the dropouts are a relatively small proportion of the entire group, the bias due to their omission is small. Because relatively few adolescents drop out before the end of tenth grade, the bias is particularly small for the eighth and tenth graders. It should also be noted that because any bias resulting from exclusion of the dropouts usually remains constant from year to year, their exclusion should introduce little or no bias in estimates of change or trends.

An issue that is relevant to the study of sensitive behaviors, such as drug use, is the extent to which respondents will answer honestly. Considerable inferential evidence suggests that the procedures used in this study produce largely valid data. This evidence includes the following points: Large proportions of respondents report using illegal substances; various drugs exhibit trends in different ways over time; there are very few missing data in response to questions on drug use, even though respondents are instructed not to answer questions they would prefer not to answer; the high correlations with other behaviors such as grades, delinquency, religious attitudes, and truancy indicate a high degree of construct validity; a high degree of consistency can be noted over time in individuals' reports (that is, the responses are reliable); and other factors that are discussed in detail in other publications (see Johnston, O'Malley, & Bachman, 2000; O'Malley, Bachman, & Johnston, 1983).

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