A PROSPECTIVE COHORT STUDY INVESTIGATING THE EXPLANATION OF SOCIOECONOMIC INEQUALITIES IN HEALTH IN THE NETHERLANDS

In this paper, the objectives, design, data-collection procedures and enrolment rates of the Longitudinal Study on Socio-Economic Health Dif ferences (LS-SEHD) are described. This study started in 1991, and is t he first large-scale longitudinal study of the explanation of socio-ec onomic inequalities in health in the Netherlands. The LS-SEHD aims at making a quantitative assessment of the contribution of different mech anisms and factors to the explanation of socio-economic inequalities i n health. It is based on a research model incorporating both 'selectio n' and 'causation' mechanisms, and a wide range of specific factors po ssibly involved in these mechanisms: health-related life-style factors , structural/environmental factors, psychosocial stress-related factor s, childhood environment, cultural factors, psychological factors, and health in childhood. The design of the LS-SEHD is that of a prospecti ve cohort study. An aselect sample, stratified by age, degree of urban ization and socio-economic status, for approx. 27,000 persons was draw n from the population registers in a region in the Southeastern part o f The Netherlands. The persons in this sample received a postal questi onnaire. An aselect subsample of approx. 3500 persons from the respond ents to the postal questionnaire was, in addition, approached for an o ral interview. The follow-up of these samples will use routinely colle cted data (mortality by cause of death, hospital admissions by diagnos is, cancer incidence), as well as repeated postal questionnaires and o ral interviews. The response rate to the base-line postal questionnair e was 70.1% (n = 18,973), and that to the base-line oral interview was 79.4% (n = 2802). If the LS-SEHD is compared to a number of frequentl y cited longitudinal studies of socio-economic inequalities in health from the United Kingdom, it appears that the differences with the OPCS Longitudinal Study and the birth cohort studies (such as the National Survey of Health and Development) are huge. The LS-SEHD is more akin to the Whitehall(I)-study and the West of Scotland 20-07 study. For ex ample it has the sample size of the former but the open population and emphasis on social factors of the latter. A comparison of the results of various longitudinal studies of socio-economic inequalities in hea lth is recommended.