2.1. Participants, design and procedure
The sample consists of primary caregivers of 8568 nine-year-old school children participating in the Growing Up in Ireland Study, a nationally representative cohort study of children living in the Republic of Ireland. The sample of children was selected through a two-stage sampling method within the primary school system. In the first stage of sampling, 1105 primary schools from the national total of 3177 were selected using a probability proportionate to size (PPS) sampling method. In the second stage, a random sample of eligible children was selected within each school. At the school level, a response rate of 82.3% was achieved, while at the level of the household (i.e. eligible child selected within the school) a total of 57% of children and their families participated in the study. The children in this sample represent approximately 1 in 7 of all nine-year old children resident in Ireland at the start of the study in 2006. The sample was weighted by adjusting the distribution of the sample to known population figures on the number and characteristics of children and their families from the 2006 Census of Population in Ireland. Characteristics accounted for in the weights included family structure, social class, economic and disadvantaged status. Trained social interviewers conducted computer assisted personal interviews with the study child and both parents/guardians (where applicable) within the home. Parents nominated a primary caregiver (the parent who spent most time with the study child) who was the primary respondent. Mothers were the primary caregiver for 98% of the study children. All stages of the Growing Up in Ireland project were subject to ethical review by the Irish Health Research Board's Research Ethics Committee.