Researchers wanted to study alcohol use by teenagers and observe if there were any differences between cohorts over time. They compared three groups of students (grades 6 to 8, grades 9 to 10, and grades 11 to 12) and focused on the frequency and amount of their alcohol use over a five-year period. They collected useful information about age-related changes, changes within the same individuals as they aged, and were able to account for the possibility of cohort and/or time of measurement effects. This is an example of .