The Women’s Health Initiative consists of two major parts: a set of randomized Clinical Trials and an Observational Study. The WHI Clinical Trials (CT) (N=68,132) includes three overlapping components, each a randomized controlled comparison among women who were postmenopausal and aged 50-79 at randomization. The three CT components are: the Hormone Therapy Trials (HT), Dietary Modification Trial (DM), and Calcium and Vitamin D Trial (CaD). Eligible women could be randomized into one, two, or all three of the CT components. Women who were ineligible or unwilling to join the CT were invited to join the Observational Study (OS) (N=93,676).
The HT Trials consist of two separate trials, one for women who were post-hysterectomy at randomization (the Estrogen-Alone Trial) and one for women with an intact uterus (the Estrogen plus Progestin Trial). In both trials there was a 1:1 randomized, double-blind allocation between active and placebo study pills. The DM Trial randomly assigned women to either a sustained low-fat eating pattern (40%) or self-selected dietary behavior (60%). The CaD component is a 1:1 randomized double-blind trial of active CaD versus placebo.
Age distribution goals were specified for the CT as follows: 10% for ages 50-54; 20% for ages 55-59; 45% for ages 60-69; 25% for ages 70-79. Another study goal was to enroll women of racial/ethnic minority groups in the same proportion as found in the general population, according to the 1990 census.
Prentice R, Rossouw J, Furberg C, Johnson S, Henderson M, Cummings S, Manson J, Freedman L, Oberman A, Kuller L, Anderson G. Design of the WHI Clinical Trial and Observational Study. Control Clin Trials 1998;19:61-109.
Anderson G, Manson J, Wallace R, Lund B, Hall D, Davis S, Shumaker S, Wang C, Stein E, Prentice R. Implementation of WHI Study Design. Ann Epidemiol 2003;13:S5-S17.