The women's health initiative observational study: baseline characteristics of participants and reliability of baseline measures

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Introduction

The Women's Health Initiative (WHI) Observational Study (OS) was established to explore the predictors and natural history of important causes of morbidity and mortality in postmenopausal women, and to serve as a secular control for the WHI Clinical Trial (CT). It enrolled 93,676 ethnically diverse women born in four different decades, from those who came of age in the depression-era, to the first members of the baby boom. Accordingly, this cohort reflects a wide range of socio-cultural influences on opportunities and health behaviors.

OS participants will contribute longitudinal data on health status, risk exposures and disease events. The follow-up interval will be slightly shorter than that in the clinical trial, approximately 7 years. All OS women had a physical examination at baseline and 3 years. Additional data are obtained with annual mailed questionnaires. These forms explore risk exposures, health behaviors, and the prevalence of less common diseases to provide a comprehensive view of both classical and novel risk factors, as well as secular trends in the predictors of healthy aging and disease events. Because of its size, the OS will permit exploration of factors associated with less common diseases.

This article describes the demographic, reproductive, dietary, and health characteristics of the OS women by ethnicity and age. In addition, we present information on the reliability of many of the baseline measures assessed in a subset of participants who were selected for the Measurement Precision Study (OS-MPS).

Section snippets

Methods

Study participants were enrolled at 40 centers throughout the United States between October 1, 1993 and December 31, 1998. Potential subjects were excluded if they did not plan to reside in the area for at least 3 years, had medical conditions predictive of survival less than 3 years, or had complicating conditions such as alcoholism, drug dependency or dementia. All participants provided informed consent using materials approved by institutional review boards at each center. Details of the

Results

93,726 women enrolled in the OS between September 1, 1994 and December 31, 1998. Of these, 31 provided insufficient baseline data to be included in these analyses, and 19 duplicate enrollments were found across multiple sites. After removing these, the remaining 93,676 women form the final analytic OS cohort, of which 78,013 (83.3%) were White, 7,639 (8.2%) Black, 3,623 (3.9%) Hispanic, 2,671 (2.9%) Asian/Pacific Islander, 422 (0.57%) American Indian, and 1308 (1.4%) of unknown race/ethnicity.

Discussion

The fraction of the US population comprised of ethnic minorities decreases with age and this is reflected in the composition of the WHI OS cohort. According to US Census data for women aged 50 to 59, 60 to 69, and 70 to 79 years, the fraction of Blacks declines from 11% to 10% to 8%, and the fraction of Hispanics from 7% to 6% to 4%, while the fraction of Whites increases from 78% to 81% to 85% (4). The trends in the OS are similar but the minority fractions are slightly lower in each decade.

Acknowledgements

The Writing Group wishes to acknowledge the contributions of Sandra A. Daugherty, M.D., M.P.H., WHI Clinical Center, University of Nevada, Reno, NV, who passed away as this manuscript was being prepared.

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