Nonresponse bias due to consent procedures in school-based, health-related research

Written, parental consent was solicited from parents of 517 seventh grade students in four junior high schools in Bergen, Norway for their children's participation in a longitudinal nutrition education study. After having received several reminders, 97.3% of the parents gave their written consent. Students whose parents gave an immediate written consent reported more positive eating habits and were more likely to plan for college than were students whose parents responded after receiving one or more reminders. The observed differences did not severely bias our baseline estimates of these variables, and the magnitude of the observed intervention impact did not change as a result of excluding students whose parents were slow to respond. Excluding these students did, however, affect our ability to identify important factors associated with healthy eating behavior including socio-economic status. Subject selection bias may represent a threat to the validity of studies using a written parental consent procedure.