Table 1 shows basic descriptive results by wave, and Table 2 shows the same information by school. Some variation is to be expected, because the waves involved different sets of schools and new cohorts of students each year. Average enrollment declined about 10% from wave 1 to wave 8, and the average number of opt-outs (survey-ineligible students) increased, probably reflecting a modest level of both institutional and individual survey fatigue. Out-choices (outdegree) averaged about 2.8 to 3.6, and declined somewhat over time. Other network statistics (reciprocity, transitivity, and 3-cycles) changed little over time. Reciprocity was lower than in comparable studies (Burk et al., 2012; Mercken et al., 2012), perhaps reflecting a broader than usual friendship criterion suggested by survey wording (“…one of my best friends…”), or the lack of a limit on number of friendship choices. Days per month participants reported spending free time with friends with no adult supervision varied across waves, trending upwards during the course of each school year.
