A total of 68,132 women were enrolled in the clinical trials. Women could enroll in one or more of the clinical trials. The final enrollment breakdown was as follows:

  • CaD study, 36,282

  • Dietary-modification trial, 48,835

  • Hormone trial, 27,347; 16,608 women were enrolled in the estrogen-plus-progestin study and 10,739 in the estrogen-alone study

In both hormone trials and in the CaD trial there was a 1:1 randomized and double-blind allocation between active and placebo study pills. In the dietary-modification trial, 40% of enrollees were assigned to the low-fat eating pattern and 60% to the comparison group.4

The observational study enrolled 93,676 women who were determined to be ineligible or unwilling to participate in the clinical trials.

The WHI Extension Study enrolled 115,400 consenting participants from each of the original WHI study components for an additional 5 years of followup, from 2005 to 2010.

Data Collection5

Data on participants’ past hormone use at baseline, with current medications and current supplements, were collected for participants in both the clinical trial and the observational study through in-person interviews. Physical measurements and blood samples were taken at baseline, and thereafter physical measurements were taken annually for clinical-trial participants and at year 3 for observational-study participants. Data on clinical-trial patients’ current medications and supplements were assessed for clinical-trial participants at years 1, 3, 6, and 9, and on observational-study participants’ medications and supplements at year 3.

Twice a year, all clinical-trial participants completed a medical-history questionnaire to assess outcomes, such as hospitalizations, heart disease, stroke, fractures, and cancer. Observational-study participants completed a medical history questionnaire annually to assess outcomes. Observational-study participants also completed a self-administered followup survey each year except study year 2. The survey was modified each year. In general, it asked participants to provide information about their weight and any changes in weight; respond to questions about health behaviors, such as physical activity, alcohol and caffeine consump-


The imbalanced randomization was set to reduce the higher staff and material costs associated with implementing the intervention while maintaining statistical power for testing hypotheses.


See the frequency-of-data collection table at http://www.whiscience.org/data/collection_frequency_grid.pdf (accessed April 8, 2010) for a more detailed description of the data that were captured and when for each study component.

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