and control groups) but only observe and measure outcomes in subjects who are (or are not, in the case of a comparison group) exposed to an intervention (for example, a smoking ban that decreases secondhand smoke exposure) (Rosenbaum, 2002). Such studies have less control of potential confounders than do experimental studies, such as randomized controlled trials, and are more prone to selection bias and to bias in the choice of comparison populations. Observational studies provide information for identifying associations and are especially useful for generating hypotheses for further testing; they are less useful for determining causality. The Nurses’ Health Study (NHS) and the Study of Women’s Health Across the Nation (SWAN) are examples of large observational studies. The NHS was originally intended to investigate the potential long-term consequences of oral contraceptives and later adapted to investigate factors that influence women’s health, especially in preventing cancer (NHS, 2008). SWAN was designed to collect information on the natural history of menopause (SWAN, 2010).

A randomized clinical trial is a prospective experiment in which investigators assign an eligible sample of people randomly to one or more treatment groups and a control group and follow subjects’ outcomes. Randomized clinical trials are usually considered the best for testing the efficacy of a treatment or intervention (Rosenbaum, 2002). The Women’s Health Study (WHS) was a randomized clinical trial in which the interventions were aspirin and vitamin E (WHS, 2009). The WHI consisted of both an observational study and three blinded, randomized clinical trial components that had hormone therapy, calcium and vitamin D, or dietary and exercise modification as the interventions (WHI, 2010). The randomized clinical trial in the WHI identified a risk of heart disease to be associated with combination estrogen hormone therapy, which was previously thought to be cardioprotective, and it confirmed the risk of breast cancer, venous thromboembolism, and stroke. Confidence in those results facilitated a decision to halt the study and led to a rapid change in prescribing practices (WHI, 2010).

Randomized clinical trials, however, have limitations of their own. Because of the expense and number of subjects needed to assess a given drug or other treatment, it is not possible to change key variables. The ability to extrapolate the results to a larger population might also be limited (Rosenbaum, 2002). In addition, ethical and practical considerations of the studies, including the ethics of placebo controls, need to be taken into account.

SUBJECT SAMPLING

If study results are to be extrapolated to the general population, the research sample needs to reflect the general population. Ensuring that research can be applied to the general population requires more than simply incorporating members of a subpopulation as part of the overall sample; it requires adequate numbers to ensure the statistical power to evaluate effects in that subpopulation. It is important to note that using gender or sex as a control variable is not the same as



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