Background Initial non-participation and loss to follow-up in the Danish youth cohort Vestliv could introduce selection bias of the measured risk estimates.

Objective To investigate the impact of initial non-participation and loss to follow-up on the validity of descriptive measures and selected estimates of relative risk.

Methods Of the 3681 young people defining the source population, 83% answered a questionnaire at baseline in 2004. At follow-up waves in 2007 and 2010, the response rates were 71% and 64%, respectively. Relative ORs (RORs) were used to examine the impact of initial non-participation and loss to follow-up on the association between socioeconomic or personal risk factors at age 14/15 and educational attainment at age 20/21. RORs were calculated as OR (baseline population)/OR (source population) or OR (follow-up population)/OR (baseline population).

Results The participants had slightly better school abilities and came more often from homes with two adults, higher income or higher educational level. These differences increased at subsequent follow-ups. The effect of initial non-participation on the ORs was modest with most RORs being close to one. Loss to follow-up led to larger variations in the RORs ranging from 0.77 to 1.62 although for most estimates, the bias was minor. None of the measured RORs were statistically different from one indicating no significant bias.

Conclusions Although certain characteristics were related to those who initially chose to participate and especially to those who participated at follow-ups, it did not have any large influence on the relative risk estimates measured in the study.