4.1.1. Data collection

The foundation for the present thesis was provided by the database of the Northern Finland 1966 Birth Cohort. Originally, the Northern Finland 1966 Birth Cohort, called nowadays the Northern Finland Health and Well-being Study, was assembled by Professor (emerita) Paula Rantakallio. She aimed to describe and analyse the risk factors for perinatal deaths and low birth weight. This database is based on the unselected, genetically homogenous general population: In the two northernmost Finnish provinces, i.e., Oulu and Lapland, 96% of all women (n = 12068), with an expected date of delivery falling between 1^st January and 31^st December, were evaluated. They gave birth to 12058 liveborn infants. The majority of the cohort members are Finns, with less than 1% being Lapps and Gypsies (nowadays often referred to as “Roma”). A variety of biological, socioeconomic and health conditions as well as living habits and family characteristics of the cohort members have been collected prospectively from pre-natal stages up to the age of 31 (Rantakallio 1969, 1988; Rantakallio et al. 1995, 1997).

4.1.2. The follow-ups

The follow-up of the cohort members has been continued until these days, and the data thus collected have regularly been linked to national registers, e.g., hospital discharge registers. Until the end of 2001 three main follow-up surveys have been conducted. The first follow-up was performed when a cohort member was one year old (1-year follow-up) in 1967. Data on growth, development and health status of the children at that time was gathered (Rantakallio 1988).

The second follow-up of the cohort (14-year follow-up) was conducted at the end of 1980 and in early 1981 by using postal questionnaires. Each child and their families were asked about the child’s health, growth, hobbies, living habits, school performance, as well as the social situation of the family and family background variables (Rantakallio 1988, Järvelin et al. 1997). The response rates in this survey reached up to 97%. By the age of 14 years, 278 cohort members had died. Thus, the number of the cohort members alive at that time was 11780 (Rantakallio 1988).

The latest follow up was conducted during 1997–98 (Sorri & Järvelin 1998). In this 31-year follow-up survey, postal questionnaires were sent to 11541 cohort members, and the response rate was 75.3%. All those, who were living in northern Finland or in the capital area, were invited to a clinical examination (n = 8463) and 71.2% attended.

The data collection procedure in the Northern Finland 1966 Birth Cohort Study and the main data used in the original publications I–IV are presented in Figure 2.