The sub-sample that attended the clinical examination at age 31 y is adequately representative of the NFBC1966 in terms of gender and socio-economic indicators at birth and at age 31 y [35]. Even better representativeness was observed when the sub-group with growth data and height SNP information (N = 3,538) was compared with attendees of clinical examination who did not have this information available (N = 2,469). In this comparison, men had data available slightly more often than women (61% vs. 57%). There were no differences regarding unemployment history or education (data available for 58–60% in all groups). There were small differences between social classes at birth (data available for 56–62% in all groups). At age 31 y, other social classes had more often data available than farmers (57–62% vs. 51%), but it has to be noted that this may be explained by random variation since the farmers group at 31 years is small (N = 214).
