Cohort Profile: The Diabetes Study of Northern California (DISTANCE)--objectives and design of a survey follow-up study of social health disparities in a managed care population.

One of the challenges of the national initiative, Healthy People 2010, is to support interventions that will reduce social disparities in health. While social disparities such as differences in education, income, race or ethnicity may affect health, the mechanisms are poorly understood. If social disparities in health originate in childhood, are current social disparities in health modifiable and are they the responsibility of a medical provider or health plan? Nonetheless, modifiable factors may exist at the individual, neighbourhood or system level that mediate (explain) social disparities in health and that may be suitable targets for interventions aiming to reduce disparities. Our aim was to survey and prospectively follow a large, diverse and wellcharacterized population with diabetes and to collect data on risk factors which may affect diabetes health outcomes but which may differ substantively in prevalence or effect size across ethnic groups or educational levels. The 2002 Institute of Medicine report, ‘Unequal Treatment: Confronting Racial and Ethnic Disparities in Health Care’, detailed the socioeconomic fragmentation of health care quality and access, and its differential, negative impact on minorities. As access to health care is an important determinant of health outcomes and is in turn associated with ethnicity and socioeconomic position, it represents a potent source of confounding bias in population-based studies of social health disparities. At the population level, identifying antecedents and determinants of social differences in disease progression remains a challenge. There are nationally representative surveys (e.g. National Health and Nutrition Examination Survey) that identify members of the general population with diabetes and that can estimate risk factors that are associated with diabetes complications, but these cohorts are often crosssectional or panel studies and have limited ability to track continuous changes in individual-level health care, outcomes and risk factors over time. Longitudinal, population-based studies often have too few minority subjects with diabetes to reveal racial or ethnic differences. The relatively uniform access to care in a managed care health plan such as Kaiser Permanente provides an advantageous setting in which to conduct a longitudinal study of social disparities in health and study findings may be compared with population-based y Additional information about DISTANCE will be published at http://distancesurvey.org * Corresponding author. Kaiser Permanente – Division of Research, 2000 Broadway, Oakland, CA 94612, USA. E-mail: Howard.H. Moffet@kp.org 1 Kaiser Permanente – Division of Research, 2000 Broadway, Oakland, CA 94612, USA. 2 Department of Psychiatry, Department of Pediatrics and Center for Health and Community, University of California, 3333 California Street, Suite 465, San Francisco, CA 94118, USA. 3 University of California, San Francisco School of Medicine, Center for Vulnerable Populations, San Francisco General Hospital, San Francisco, CA 94110, USA. 4 Department of Medicine and Center for Health and Community, University of California, 3333 California Street, Suite 465, San Francisco, CA 94118, USA. 5 Division of Biostatistics, School of Public Health, University of California, Berkeley, CA 94704, USA. 6 Department of Epidemiology, School of Public Health & Community Medicine, University of Washington, Seattle, WA 98195, USA. Published by Oxford University Press on behalf of the International Epidemiological Association