Data were collected immediately prior to, during (eight months into the intervention) and at the end of the intervention (after 20 months). During the sample period, a number of children left the study, for example were moved by their parents to another preschool or became eligible for primary schooling. Child drop-out due to these reasons was non-random across the intervention and control-groups. Indeed, some preschools had no children that were measured at all three data-collection points and some children could not be followed throughout all three data-collection periods. Drop-out was mainly because children became eligible for primary school and subsequently left the preschool. Only a negligible number of children were taken out of the participating preschools before they become eligible for primary schooling. For clarity of the entire drop-out process