Cohort Studies

Cohort studies are often the next step in building the strength of evidence regarding an association between an exposure and an outcome. Cohort studies typically look forward in time (i.e., prospective studies) and are generally more expensive and take longer to complete than case-control studies. However, they provide a more accurate estimate of the relative risk for women who take HRT and those who do not. A cohort study is also an observational study—one that observes outcomes in groups but does not assign participants to a particular exposure or treatment. In a cohort study of HRT and CHD, a researcher would identify a group of women taking HRT and a similar group of women who have chosen not to take HRT, and the researcher would then follow them over time and count the number of CHD events. Because outcome events may be uncommon in each group and may take many months to occur, cohort studies often require large numbers of participants and long follow-up periods to show significant differences between groups.

The primary statistical measure from a cohort study is relative risk. This is a ratio of the rate of CHD events among women who choose to take HRT divided by the rate among women who choose not to take HRT. A common form of bias in cohort studies related to prevention is the healthy user bias, when participants who choose one preventive measure (e.g., HRT) also tend to make healthier lifestyle decisions (e.g., diet, exercise) that may also prevent the measured outcome (i.e., CHD).

Beginning with case-control studies and then using larger cohort studies, observational research showed that HRT might reduce the incidence of CHD, fractures, and colorectal cancer. These observational studies also suggested that the same therapy might cause harm, with a slightly increased risk of breast cancer, stroke, and venous thromboembolism. On balance, however, even a small positive impact of HRT on preventing CHD was thought to far outweigh the potential adverse effects of HRT.

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