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Observational Study N = 93,676Observational Study (OS)


The Observational Study (OS) component of the WHI complements the Clinical Trial (CT) by assessing new risk indicators and biomarkers for disease in a large prospective cohort of 93,676 postmenopausal women. The OS cohort is comprised of women who were either ineligible or unwilling to participate in the CT. Over 16% of OS participants were members of a racial / ethnic minority group (see demographic and participant characteristics in Baseline Data). OS enrollment ran from 1994 through 1998. OS women were followed for 6 - 10 years, depending on when they enrolled in the study. See Screening and Inclusion/Exclusion Criteria and Randomization and Enrollment in the Screening and Enrollment section for details on WHI screening and enrollment procedures.

The major clinical outcomes of interest in the OS are coronary heart disease, stroke, breast cancer, colorectal cancer, endometrial cancer, ovarian cancer, osteoporotic fractures, diabetes, and total mortality. Most outcomes were initially ascertained by self-report on an annual questionnaire and documented by medical records. Charts with potential cardiovascular, cancer, and fracture outcomes were sent to physician adjudicators for evaluation and classification. Staff at the Clinical Coordinating Center coded and adjudicated all cancers of major interest in the study using standardized SEER guidelines. Refer to Outcomes for more details.

A 1% sample of OS participants were asked to return to the clinic 1 - 3 months after their baseline and 3-year visits for a reliability study, at which time blood was drawn and selected data item prone to measurement error were repeated. Details on the OS reliability study are described in the OS Baseline Monograph.


Routine follow-up activities consisted of mailings sent annually from the CCC, and a Clinical Center visit at 3 years after enrollment to update selected baseline data, obtain additional risk factor data, and collect a blood specimen. Annual mailings consisted of a cover letter, a self-administered Medical History Update to assess health outcomes, and a self-administered exposure questionnaire. Non-responders received two additional mailings and telephone contacts by clinical center staff. When these efforts were not successful, clinical center staff contacted proxies to determine the location and status of the participant and to collect information on health events. Final data were collected by mail during the OS close-out period, April 2004 to March 2005. Refer to Data Variables by Form to view the annual forms and Frequency of data collection​​​ for the schedule.

The annual follow-up response rate was over 94% each year. At the year 3 clinic visit, 96% completed medical history updates and 83% provided blood samples. A participant was considered lost-to-follow-up if there was no outcomes information from the participant for 24 months. At the end of the close-out period, 4.1% were either lost-to-follow-up or had stopped follow-up, and 6.1% were deceased.

Related publications

Baseline monograph

See Also