Overview - The Observational Study (OS) enrolled 93,676 women who were either ineligible or unwilling to join one of the WHI Clinical Trials. This component of the WHI assesses new risk indicators and biomarkers for disease. Over 16% of OS participants at enrollment were members of a racial / ethnic minority group. Enrollment for the OS began in 1994 and ended in 1998. OS women were followed for between 6 and 10 years, depending on when they enrolled in the study.