Figure 1 presents the retrieval and response rates
throughout the study. Lack of identification of the kindergarten
of the child and refusal of the school to collaborate
were the major reasons for missing data at age 3 (34 % of
the original cohort) as well as at age 5 (27 %). When
children were retrieved at school, the majority of the parents
completed the questionnaire (84 % at age 3 and 91 % at age
5) and respectively 89 and 96 % of the children were
examined. Therefore, most of the missing data could be
categorised as ‘missing at random’.