During an assessment, study members are brought back to Dunedin from wherever in the world they live. They participate in a day of interviews, physical tests, dental examinations, blood tests, computer questionnaires and surveys. Sub-studies of the Dunedin Study include the Family Health History Study which involved the parents of Dunedin Study members to find out about the health of family members (2003–2006); the ongoing Parenting Study which focuses on the Dunedin Study member and their first three-year-old child; and the Next Generation Study which involves the offspring of Dunedin Study members as they turn 15 and looks at the lifestyles, behaviours, attitudes and health of today's teenagers, and aims to see how these have changed from when the original Study Members were 15 (in 1987–88). This means that information across three generations of the same families will be available. Great emphasis is placed on retention of study members. At the most recent (age 38) assessments, 96% of all living eligible study members, or 961 people, participated. This is unprecedented for a longitudinal study, with many others worldwide experiencing 20–40% drop-out rates.